I have created the code in my way. There will be many simple ways to create a code.So, Just ignore it and try to understand my code.

DetailsActivity.java

package trickandroid.fulllogin;

import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;

import com.firebase.ui.database.FirebaseListAdapter;
import com.github.paolorotolo.expandableheightlistview.ExpandableHeightListView;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;

import static trickandroid.fulllogin.R.id.conRaCaNumET;

public class DetailsActivity extends AppCompatActivity {

    private static final String TAG = "DetailsActivity";

    //variables
    private TextView nameTV,numberTV, areaTV, doorNumTV,curAreaTV,curNumTV;
    private String userID, key, areaName,name,number,due;
    private ExpandableHeightListView expandableListView;
    private Button editBTN,shiftBTN,disBTN, okBTN, cancelBTN;
    private EditText detailNumberET, detailNameET, dueET, doorET, phoneET, aatharET, rationET, doorNumET;
    private LinearLayout detailLL, buttonLL, editBTNLL, doorNumLL,curNumLL,curAreaLL;
    private Spinner addConSpinner;
    private String newDoorNum;

    //Firebase Variables
    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;
    private FirebaseDatabase firebaseDatabase;
    private DatabaseReference databaseReference;
    private DatabaseReference mdata;
    private DatabaseReference disconnected;
    private DatabaseReference connection;
    private DatabaseReference areaList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_details);

        FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
        userID = user.getUid();

        firebaseDatabase = FirebaseDatabase.getInstance();
        databaseReference = firebaseDatabase.getReference();

        mAuth = FirebaseAuth.getInstance();

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    Log.d(TAG, "onAuthStateChanged: " + userID);
                    // User is signed in
                    Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
                } else {
                    // User is signed out
                    Log.d(TAG, "onAuthStateChanged:signed_out");
                }
                // ...
            }
        };

        areaList = FirebaseDatabase.getInstance().getReference().child("users").child(userID).child("AreaList");
        connection = FirebaseDatabase.getInstance().getReference().child("users").child(userID).child("Connection List");
        mdata = FirebaseDatabase.getInstance().getReference().child("users").child(userID).child("Connection Detail");
        disconnected = FirebaseDatabase.getInstance().getReference().child("users").child(userID).child("Disconnected");

        due = getIntent().getExtras().getString("due");
        key = getIntent().getExtras().getString("key");
        Log.d(TAG, "onCreate: " + key);
        areaName = getIntent().getExtras().getString("areaname");
        Log.d(TAG, "onCreate: " + areaName);
        name = getIntent().getExtras().getString("name");
        number = getIntent().getExtras().getString("number");

        nameTV = (TextView) findViewById(R.id.nameTV);
        Log.d(TAG, "onCreate: " + name);
        nameTV.setText(name);
        numberTV = (TextView) findViewById(R.id.numberTV);
        Log.d(TAG, "onCreate: " + number);
        numberTV.setText(number);
        areaTV = (TextView) findViewById(R.id.areaTV);
        areaTV.setText(areaName);

        addConSpinner = (Spinner) findViewById(R.id.spinnerArea);
        final FirebaseListAdapter<String> firebaseListAdapter = new FirebaseListAdapter<String>(DetailsActivity.this,
                String.class,
                android.R.layout.simple_list_item_1,
                areaList) {
            @Override
            protected void populateView(final View v, final String name, int position) {
                TextView textView = (TextView) v.findViewById(android.R.id.text1);
                textView.setText(name);
            }
        };
        addConSpinner.setAdapter(firebaseListAdapter);

        expandableListView = (ExpandableHeightListView) findViewById(R.id.expandable_listview);

        // This actually does the magic
        expandableListView.setExpanded(true);

        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                showData(dataSnapshot);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        editBTN = (Button) findViewById(R.id.editBTN);
        shiftBTN = (Button) findViewById(R.id.shiftBTN);
        disBTN = (Button) findViewById(R.id.disconnectBTN);

        curAreaTV = (TextView) findViewById(R.id.CurAreaTV);
        curAreaTV.setText(areaName);
        curNumTV = (TextView) findViewById(R.id.CurNumTV);
        curNumTV.setText(number);
        doorNumTV = (TextView) findViewById(R.id.doorNUmTV);
        doorNumET = (EditText) findViewById(R.id.doorNumET);
        detailNumberET = (EditText) findViewById(R.id.detailNumberET);
        detailNameET = (EditText) findViewById(R.id.detailNameET);
        dueET = (EditText) findViewById(R.id.dueET);
        doorET = (EditText) findViewById(R.id.detailDoorET);
        phoneET = (EditText) findViewById(R.id.conPhNumET);
        aatharET = (EditText) findViewById(R.id.conAthrNumET);
        rationET = (EditText) findViewById(conRaCaNumET);
        detailLL = (LinearLayout) findViewById(R.id.detailLL);
        buttonLL = (LinearLayout) findViewById(R.id.buttonLL);
        editBTNLL = (LinearLayout) findViewById(R.id.editBTNLL);
        doorNumLL = (LinearLayout) findViewById(R.id.doorNUMLL);
        curAreaLL = (LinearLayout) findViewById(R.id.curAreaLL);
        curNumLL = (LinearLayout) findViewById(R.id.curNumLL);

        disBTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });

        okBTN = (Button) findViewById(R.id.detailOKBTN);
        cancelBTN = (Button) findViewById(R.id.detailCancelBTN);

        shiftBTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                shiftBUTTON();
            }
        });

        okBTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (detailLL.getVisibility()==View.VISIBLE){
                    final String detailDue = dueET.getText().toString();

                    String doorNum = doorET.getText().toString();
                    doorNum = "Door Number = " + doorNum;

                    String conName = detailNameET.getText().toString();
                    conName = "Name = " + conName;

                    final String name = detailNameET.getText().toString();

                    String phoneNum = phoneET.getText().toString();
                    phoneNum = "Phone Number = " + phoneNum;

                    String aadharNum = aatharET.getText().toString();
                    aadharNum = "Aadhar Number = " + aadharNum;

                    String rationNum = rationET.getText().toString();
                    rationNum = "Ration Number = " + rationNum;

                    if (name.isEmpty()){
                        toast(getString(R.string.enter_name));
                    } else if (detailDue.isEmpty()){
                        toast(getString(R.string.enter_due_amount));
                    } else if (doorNum.isEmpty()){
                        toast(getString(R.string.enter_door_number));
                    } else if (phoneNum.isEmpty()){
                        toast(getString(R.string.enter_phone_number));
                    } else if(aadharNum.isEmpty()){
                        toast(getString(R.string.enter_aathar_number));
                    } else if (rationNum.isEmpty()){
                        toast(getString(R.string.enter_ration_number));
                    } else {

                        AlertDialog.Builder alertDialog = new AlertDialog.Builder(DetailsActivity.this);

                        // Setting Dialog Message
                        alertDialog.setMessage(R.string.yes_dialouge);

                        // Setting Positive "Yes" Button
                        final String finalConName = conName;
                        final String finalDoorNum = doorNum;
                        final String finalPhoneNum = phoneNum;
                        final String finalAadharNum = aadharNum;
                        final String finalRationNum = rationNum;
                        alertDialog.setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int which) {
                                ConnectionInformation cInfo = new ConnectionInformation(detailDue, areaName,number, finalDoorNum, finalConName, finalPhoneNum, finalAadharNum, finalRationNum);
                                mdata.child(areaName).child(key).setValue(cInfo);

                                Connections connect = new Connections(number, name, detailDue);
                                connection.child(areaName).child(key).setValue(connect);

                                toast(getString(R.string.edited_successfully));

                                nameTV.setText(name);

                                cancelMethod();
                            }
                        });

                        // Setting Negative "NO" Button
                        alertDialog.setNegativeButton(R.string.no, new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,	int which) {
                                dialog.cancel();
                            }
                        });

                        // Showing Alert Message
                        alertDialog.show();
                    }
                } else {

                    final String newNum = detailNumberET.getText().toString();
                    newDoorNum = "Door Number = " + doorNumET.getText().toString();
                    final String door =doorNumET.getText().toString();
                    final String mAreaName = addConSpinner.getSelectedItem().toString();

                    if (newNum.isEmpty()){
                        toast(getString(R.string.enter_new_number));
                    } else {

                        AlertDialog.Builder alertDialog = new AlertDialog.Builder(DetailsActivity.this);

                        // Setting Dialog Message
                        alertDialog.setMessage(R.string.shift_dialouge);

                        // Setting Positive "Yes" Button
                        alertDialog.setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int which) {
                                databaseReference.addValueEventListener(new ValueEventListener() {
                                    @Override
                                    public void onDataChange(DataSnapshot dataSnapshot) {
                                        for (DataSnapshot ds : dataSnapshot.getChildren()){

                                            if (areaName.equals(mAreaName)){
                                                ConnectionInformation cInfor = new ConnectionInformation();
                                                cInfor.setPhoneNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getPhoneNum());
                                                cInfor.setAadharNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getAadharNum());
                                                cInfor.setRationNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getRationNum());
                                                ConnectionInformation info = new ConnectionInformation(due,mAreaName,newNum,newDoorNum,name,cInfor.getPhoneNum(),cInfor.getAadharNum(),cInfor.getRationNum());
                                                mdata.child(mAreaName).child(key).setValue(info);

                                                Connections connect = new Connections(newNum,name,due);
                                                connection.child(mAreaName).child(key).setValue(connect);

                                                toast(getString(R.string.shifted_successfully));

                                                areaTV.setText(mAreaName);

                                                numberTV.setText(newNum);

                                                doorNumTV.setText(door);

                                                cancelMethod();
                                            } else {
                                                ConnectionInformation cInfor = new ConnectionInformation();
                                                cInfor.setPhoneNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getPhoneNum());
                                                cInfor.setAadharNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getAadharNum());
                                                cInfor.setRationNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getRationNum());
                                                ConnectionInformation info = new ConnectionInformation(due,mAreaName,newNum,newDoorNum,name,cInfor.getPhoneNum(),cInfor.getAadharNum(),cInfor.getRationNum());
                                                mdata.child(mAreaName).child(key).setValue(info);

                                                Connections connect = new Connections(newNum,name,due);
                                                connection.child(mAreaName).child(key).setValue(connect);

                                                areaTV.setText(mAreaName);

                                                numberTV.setText(newNum);

                                                doorNumTV.setText(door);

                                                toast(getString(R.string.shifted_successfully));

                                                cancelMethod();

                                                Intent i = new Intent(DetailsActivity.this,DisconnectActivity.class);
                                                i.putExtra("key",key);
                                                i.putExtra("oldarea",areaName);
                                                i.putExtra("name",name);
                                                i.putExtra("oldnumber",number);
                                                i.putExtra("newArea",mAreaName);
                                                i.putExtra("newNumber",newNum);
                                                startActivity(i);

                                                moveFirebaseRecord(mdata.child(areaName).child(key),disconnected.child(areaName).child(key));

                                            }
                                        }
                                    }

                                    @Override
                                    public void onCancelled(DatabaseError databaseError) {

                                    }
                                });
                            }
                        });

                        // Setting Negative "NO" Button
                        alertDialog.setNegativeButton(R.string.no, new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,	int which) {
                                dialog.cancel();
                            }
                        });

                        // Showing Alert Message
                        alertDialog.show();
                    }

                }
            }
        });

        cancelBTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                cancelMethod();
            }
        });

        editBTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                editMethod();
            }
        });

    }

    private void moveFirebaseRecord(DatabaseReference child, final DatabaseReference child1) {
        child.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                child1.setValue(dataSnapshot.getValue(), new DatabaseReference.CompletionListener() {
                    @Override
                    public void onComplete(DatabaseError databaseError, DatabaseReference databaseReference) {

                        if (databaseError != null)
                        {
                            toast(getString(R.string.error));
                        }
                        else
                        {
                            toast(getString(R.string.shifted_successfully));
                        }
                    }
                });
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

    private void shiftBUTTON() {
        curAreaLL.setVisibility(View.VISIBLE);
        curNumLL.setVisibility(View.VISIBLE);
        areaTV.setVisibility(View.GONE);
        addConSpinner.setVisibility(View.VISIBLE);
        doorNumLL.setVisibility(View.VISIBLE);
        expandableListView.setVisibility(View.GONE);
        numberTV.setVisibility(View.GONE);
        detailNumberET.setVisibility(View.VISIBLE);
        editBTNLL.setVisibility(View.GONE);
        buttonLL.setVisibility(View.VISIBLE);
    }

    private void editMethod() {
        nameTV.setVisibility(View.GONE);
        detailNameET.setVisibility(View.VISIBLE);

        expandableListView.setVisibility(View.GONE);
        detailLL.setVisibility(View.VISIBLE);

        editBTNLL.setVisibility(View.GONE);

        buttonLL.setVisibility(View.VISIBLE);
    }

    private void cancelMethod() {
        curAreaLL.setVisibility(View.GONE);
        curNumLL.setVisibility(View.GONE);
        doorNumET.setVisibility(View.GONE);
        doorNumTV.setVisibility(View.VISIBLE);
        areaTV.setVisibility(View.VISIBLE);
        addConSpinner.setVisibility(View.GONE);
        detailNumberET.setVisibility(View.GONE);
        numberTV.setVisibility(View.VISIBLE);
        nameTV.setVisibility(View.VISIBLE);
        detailNameET.setVisibility(View.GONE);

        expandableListView.setVisibility(View.VISIBLE);
        detailLL.setVisibility(View.GONE);

        editBTNLL.setVisibility(View.VISIBLE);

        buttonLL.setVisibility(View.GONE);
    }

    private void showData(DataSnapshot dataSnapshot) {
        if (expandableListView.getVisibility()==View.VISIBLE) {
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                ConnectionInformation cInfo = new ConnectionInformation();
                cInfo.setDoorNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getDoorNum());
                cInfo.setPhoneNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getPhoneNum());
                cInfo.setAadharNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getAadharNum());
                cInfo.setRationNum(ds.child(userID).child("Connection Detail").child(areaName).child(key).getValue(ConnectionInformation.class).getRationNum());

                ArrayList<String> users = new ArrayList<>();
                users.add(cInfo.getDoorNum());
                users.add(cInfo.getPhoneNum());
                users.add(cInfo.getAadharNum());
                users.add(cInfo.getRationNum());

                ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, users);
                expandableListView.setAdapter(adapter);
            }
        }
    }

    public void toast(String m) {
        Toast.makeText(this, m, Toast.LENGTH_SHORT).show();
    }
}