The amount of rotation uses something called ‘Quaternion’. Set one up like so:
newRotation = Quaternion.Euler(0,0,90);
The 3 values are x,y,z. In the example, it rotates 90 degress on the z axis.
Then to animate this rotation, in the Update() function:
transform.rotation = Quaternion.Lerp (transform.rotation,newRotation,Time.deltaTime * someSpeedVar);
You can also use ‘Slerp’ instead of ‘Lerp’ if the rotation looks weird, but lerp has better performance.