Unity UI Two State button. Button state will reamin pressed you click on the it.

SpirteSwap

using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

[RequireComponent(typeof(Button))]
[RequireComponent(typeof(Image))]
public class SpriteSwap : MonoBehaviour
{
    [Header("Button Start State")]
    public bool isPressed;
    private Sprite stateNormal;
    private Button btnCurrent;
    private Image imgCurrent;
    private EventSystem myEventSystem;

    protected void Start()
    {
        btnCurrent      = GetComponent<Button>();
        imgCurrent      = GetComponent<Image>();
        stateNormal     = imgCurrent.sprite;//storing current sprite.
        myEventSystem   = FindObjectOfType<EventSystem>();
    }

    public virtual void OnClick()
    {
        isPressed = !isPressed;

        if (isPressed)
        {
            Debug.Log("Button pressed");
            imgCurrent.sprite = btnCurrent.spriteState.pressedSprite;
        }
        else
        {
            Debug.Log("Button not pressed");
            imgCurrent.sprite = stateNormal;
            myEventSystem.SetSelectedGameObject(null);//Deselect the button so it colour changed to normal.
        }
    }
}