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;

public class Example : MonoBehaviour
{
    // Interpolates rotation between the rotations
    // of from and to.
    // (Choose from and to not to be the same as
    // the object you attach this script to)

    public Transform from;
    public Transform to;
    float speed = 0.1f;
    float t = 0;
    void Update()
    {
        t += Time.deltaTime * speed;
        transform.rotation = Quaternion.Lerp(from.rotation, to.rotation, t);
    }
}

See Also: Slerp. LerpUnclamped.