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.

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);
    }
}