Quaternion.Lerp

Description

Interpolates between a and b by t and normalizes the result afterwards. The parameter t is clamped to the range [0, 1].

This is faster than Slerp but looks worse if the rotations are far apart.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform from;
    public Transform to;
    public float speed = 0.1F;
    void Update() {
        transform.rotation = Quaternion.Lerp(from.rotation, to.rotation, Time.time * speed);
    }
}

See Also: Slerp. LerpUnclamped.