Rotating an Object in Update Function

I originally was using:
But it rotated the object…forever. I want to rotate my object 90 degrees left and then STOP the rotation. But transform.Rotate just continued it because it was in an Update function. How can I stop the rotation at 90 degrees.
Do I need to use a Quaternion? I’ve tried learning about Quaternions on Unity Scripting API but can’t seem to get my head around them - they seem more complicated than what I want to do.
Anyway, thank you if you know the answer to this!

How about using a coroutine, this is a rotate around method that will stop after the given number of degrees has been achieved:

bool isRotating = false;
public Vector3 this_Axis;
public float deg;
public float time;

void Update () {
	if(Input.GetKey(KeyCode.Q) && !isRotating){
		StartCoroutine(RotateAround(this_Axis, deg, time));

IEnumerator RotateAround(Vector3 axis, float degrees, float duration){
	if(axis == || degrees == 0){
		return true;
	isRotating = true;
	float d = 0;
	Quaternion q;
	Quaternion startRot = transform.rotation;
	float progress = 0;	
	while(progress <= 1){
		d = Mathf.Lerp(0, degrees, progress);
		q = Quaternion.AngleAxis(d, axis);
		transform.rotation = startRot*q;
		progress += Time.deltaTime/duration;
		yield return null;
	transform.rotation = startRot*Quaternion.AngleAxis(degrees, axis);
	isRotating = false;

Hope that helps!