Quaternion.RotateTowards

Description

Rotates a rotation from towards to.

The from quaternion is rotated towards to by an angular step of maxDegreesDelta (but note that the rotation will not overshoot). Negative values of maxDegreesDelta will move away from to until the rotation is exactly the opposite direction.

		// The object whose rotation we want to match.
	var target: Transform;
	
	// Angular speed in degrees per sec.
	var speed: float;
	
	function Update () {    
	    // The step size is equal to speed times frame time.
	    var step = speed * Time.deltaTime;
	    
	    // Rotate our transform a step closer to the target's.
	    transform.rotation = Quaternion.RotateTowards(transform.rotation, target.rotation, step);
	}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform target;
    public float speed;
    void Update() {
        float step = speed * Time.deltaTime;
        transform.rotation = Quaternion.RotateTowards(transform.rotation, target.rotation, step);
    }
}