Quaternion.Slerp

public static Quaternion Slerp (Quaternion a, Quaternion b, float t);

Description

Spherically interpolates between a and b by t. The parameter t is clamped to the range [0, 1].

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.Slerp(from.rotation, to.rotation, Time.time * speed);
    }
}

See Also: Lerp, SlerpUnclamped.