AccountTrigger

trigger AccountTrigger on Account (after update) {
    //prepare an empty Map of ID-to-Account for your updates
    Map<ID, Account> acctMapToUpdate = new Map<Id, Account>();
    
    //prepare a for-loop you can use to compare old and new field values 
    for(Integer i = 0 ; i < trigger.new.size() ; i++){
        Account old = trigger.old[i];
        Account nw = trigger.new[i];
        
        //Evaluate Custom Settings to decide whether to execute actions 
        //if(Org_Exceptions__c.getOrgDefaults().Bypass_Triggers__c){//Returns only the Organization values, regardless of user
        //if(Org_Exceptions__c.getInstance().Bypass_Triggers__c){//Returns values based on User and User's Profile
        //if(Feature_Toggles__c.getValues('AccountTrigger').Activated__c){}//Returns setting for specific toggle record
            //Check if the value of TickerSymbol changed
            if(old.TickerSymbol != nw.TickerSymbol){
                //Open the account record for editing and change the Rating field 
                Account a = new Account(Id = nw.Id, Rating = 'Needs Evaluation');
                //Add the account record to your map
                acctMapToUpdate.put(nw.Id, a);
            }            
    }
    //Check if your Map has any records
    if(acctMapToUpdate.size() > 0){
        //update the account records in your map
        update acctMapToUpdate.values();
    }
}

AccountTrigger practice

trigger AccountTrigger on Account (after update) {
    //prepare an empty Map of ID-to-Account for your updates
    
    //prepare a for-loop you can use to compare old and new field values
    //start for-loop

            //Check if the value of TickerSymbol changed
                //Open the account record for editing and change the Rating field 
                //Add the account record to your map
    //end for-loop

    //Check if your Map has any records

    //update the account records in your map

}

TestAccountTrigger

@isTest
public class TestAccountTrigger {
    
    @testSetup
    static void initData(){
        Account a0 = new Account(Name = 'Account Zero', Industry = 'Finance');
        insert a0;
        
        Account a1 = new Account(Name = 'Account One', Industry = 'Finance');
        insert a1;
        
        /* These require you to create Custom Settings and their Custom Fields before you can save it in Apex
        Feature_Toggles__c toggles = new Feature_Toggles__c(Name = 'AccountTrigger', Activated__c = True);
        insert toggles;

        Org_Exceptions__c exception1 = new Org_Exceptions__c(Bypass_Triggers__c = True);
        insert exception1;
        */
    }
    
    static testMethod void testAll(){
        List<Account> getAccts = [Select ID from Account];
        
        List<Account> acctsToUpdate = new List<Account>();
            
        acctsToUpdate.add(new Account(Id = getAccts[0].Id, TickerSymbol = 'aZero'));
        acctsToUpdate.add(new Account(Id = getAccts[1].Id, TickerSymbol = 'aOne'));
        
        update acctsToUpdate;
        
        List<Account> updatedAccts = [Select ID, Rating from Account];
        for(Account a : updatedAccts){
            System.assertEquals('Needs Evaluation', a.Rating, 'This was supposed to match if TickerSymbol changed');
        }
    }
    static testMethod void negativeTest(){
        List<Account> getAccts = [Select ID from Account];
        
        List<Account> acctsToUpdate = new List<Account>();
            
        acctsToUpdate.add(new Account(Id = getAccts[0].Id, Industry = 'Banking'));
        acctsToUpdate.add(new Account(Id = getAccts[1].Id, Industry = 'Banking'));
        
        update acctsToUpdate;
        
        List<Account> updatedAccts = [Select ID, Rating from Account];
        for(Account a : updatedAccts){
            System.assertNotEquals('Needs Evaluation', a.Rating, 'This was supposed to match only if TickerSymbol changed');
        }
    }
}