Update и нажатие кнопки мыши

transform.rotation = Quaternion.Lerp(.... и т.д

Input.GetKeyDown(KeyCode.Mouse0)

Input.GetMouseButtonDown(0)

GetButtonDown

GetMouseButton

Quaternion targetRotation;

void Start()
{
   targetRotation = transform.rotation;
}

if (Input.GetMouseButtonDown(0))
   targetRotation *= Quaternion.AngleAxis(90, Vector3.up);

void Update ()
{
    //...
    transform.rotation = Quaternion.RotateTowards(transform.rotation, targetRotation, Time.deltaTime * Speed);
}

void Update()
{
transform.rotation = Quaternion.Lerp(Quaternion.identity, Quaternion.Euler(0f, 0f, -90f), Time.time);
}

using UnityEngine;
 
public class Rotate : MonoBehaviour
{
    public float Speed = 100;
 
    private Quaternion targetRotation;
 
    void Start ()
    {
        targetRotation = transform.rotation;
    }
    
    void Update ()
    {
        transform.rotation = Quaternion.RotateTowards(transform.rotation, targetRotation, Time.deltaTime * Speed);
    }
 
    void OnMouseDown()
    {
        targetRotation *= Quaternion.AngleAxis(90, Vector3.up);
    }
}

Quaternion.Angle(transform.rotation, targetRotation);

    void OnMouseDown()
    {
        if (Quaternion.Angle(transform.rotation, targetRotation) < 1)
                targetRotation *= Quaternion.AngleAxis(90, Vector3.up);
    }