You can either use the animation editor to make an opening animation, or you can use a simple script. I suggest doing it both ways, just so you have a good idea of how the two methods work. For the former, see the "Animation View" section in the reference manual. For the former, you'll have an Update() method which (assuming the door is supposed to be open) gets the rotation out of the transform, uses Quaternion.RotateTowards with a "toRotation" corresponding to the open rotation and a "maxDegreesDelta" formed by multiplying Time.deltaTime with your desired speed, and puts the resultant quaternion into transform.rotation.