How do i rotate an object 90 or -90 degrees upon collision?

     public GameObject rotatedObject;
     bool rotating = false;
     public float smoothTime = 5.0f; //rotate over 5 seconds
 
     void OnCollisionEnter(Collision col)
     {
         if (col.gameObject.name == "Arrow" || col.gameObject.name == "Fire Arrow" && !rotating) // we dont want to call this if the object is already rotating, so we check if it is
         {
             //Rotate rotatedObject by 90 degrees on the Y axis
             rotating = true;
             float rando = Random.Range(0, 100); // pick a random number between 1 and 100
             int multiplier = 1;
             if (rando > 50)
             {
                 multiplier = -1;
             }
             StartCoroutine(RotateOverTime(rotatedObject.transform.localEulerAngles.y, rotatedObject.transform.localEulerAngles.y + (90 * multiplier), smoothTime);
         }
     }
     IEnumerator RotateOverTime(float currentRotation, float desiredRotation, float overTime)
     {
         float i = 0.0f;
         while (i <= 1)
         {
             rotatedObject.transform.localEulerAngles = new Vector3(rotatedObject.transform.localEulerAngles.x, Mathf.Lerp(currentRotation, desiredRotation, i), rotatedObject.transform.localEulerAngles.z);
             i += Time.deltaTime / overTime;
             yield return null;
         }
         yield return new WaitForSeconds(overTime);
         rotating = false; // no longer rotating
     }

 public  bool turning;
 private float targetRot_y;
 public  float turnSpeed = 1f;
 
 public GameObject rotatedObject;
 
  void OnCollisionEnter (Collision col)
      {
          if (col.gameObject.name == "Arrow" || col.gameObject.name == "Fire Arrow")
          {
              int rn = Random.Range(0,2);
      if(rn == 0) {
           targetRot_y = 90;
      }
      else {
           targetRot_y = -90;
      }
      if(turning == false) 7
           StartCoroutine("Turn");
      }
 
 private IEnumerator Turn() {
      turning = true;
      Quaternion rot_old= rotatedObject.transform.rotation;
         rotatedObject.transform.Rotate(0, targetRot_y, 0);
         Quaternion rot_new = rotatedObject.transform.rotation;
        
         for (t = 0.0f; t <= 1.0f; t += (turnSpeed * Time.deltaTime)) {
             rotatedObject.transform.rotation = Quaternion.Slerp(rot_old, rot_new , t);
             yield return null;
         }
        
         rotatedObject.transform.rotation = rot_new ;
      turning = false;
 
 }

As you can see from the answers, there are many ways to do this.

     //public GameObject rotatedObject;
     void OnCollisionEnter (Collision col)
     {
         if (col.gameObject.name == "Arrow" || col.gameObject.name == "Fire Arrow")
         {
             //Rotate rotatedObject by 90 degrees on the Y axis
             rotateLerpTime = 0;
             rotateStart = transform.localRotation;
             rotateTarget = Quaternion.Euler(transform.localRotation.eulerAngles.x, 
                                             transform.localRotation.eulerAngles.y + (Random.Range(0,2) == 0 ? 90 : -90),
                                             transform.localRotation.eulerAngles.z);    
 
 
         }
     }
 
     float rotateLerpTime;
     Quaternion rotateStart, rotateTarget;
     void Update()
     {
         //transform.position += transform.forward * Time.deltaTime;
 
         if (!Mathf.Approximately(transform.localRotation.eulerAngles.y, rotateTarget.eulerAngles.y))
         {
             rotateLerpTime = Mathf.Clamp(rotateLerpTime + Time.deltaTime, 0f, 1f);
             transform.localRotation = Quaternion.Slerp(rotateStart, rotateTarget, rotateLerpTime);
         }
     }