Smooth rotation problem

I can't figure out how to create a smooth rotation, when patroling object reaches turning point. At the moment it just snaps to that direction.

 private void Update()
     {
         transform.position = Vector3.MoveTowards(transform.position, waypoints[startIndex].position, Time.deltaTime * speed);
         if(transform.position == waypoints[startIndex].position)
         {
             ChangeDestination();
         }
         ChangeRotation();
     }
     void ChangeDestination()
     {
         if (reverse)
         {
             DetermineDirection();
         }
         startIndex = nextIndex;
         nextIndex = (startIndex + direction) % waypoints.Length;
 
         if(nextIndex < 0)
         {
             nextIndex += waypoints.Length;
         }
     }
     void DetermineDirection()
     {
         if(nextIndex == 0 || nextIndex == waypoints.Length - 1)
         {
             direction *= -1;
         }
         transform.up = waypoints[nextIndex].position - transform.position;
 
         Vector2 dir = (waypoints[nextIndex].position - transform.position).normalized;
         angle = Mathf.Atan2(dir.y, dir.x) * Mathf.Rad2Deg - 90;
         newRot = Quaternion.AngleAxis(angle, Vector3.forward);
     }
     void ChangeRotation()
     {
         
         transform.rotation = Quaternion.Slerp(transform.rotation, newRot, 0.1f);
     }