How to rotate an object on a fixed angle?

Hi, I want to rotate a object on a fixed 90degree angle on a mouse click. This is the script I’m using currently.

function Update () {
if (Input.GetButton(“Fire1”)){
transform.RotateAround(Vector3.up,Time.deltaTime*100);
}
}

Any help shall be appreciated. Thanks.

Rotating an object 90 around tis own pointing up vector

function Update() 
{
   if (Input.GetButton("Fire1"))
       transform.RotateAround (transform.position, transform.up, 90);
}