If you pre ordered the game you should have a couple of Batman skins and a Batmobile skin. Now you do need to have extra skins in order to do this. How To Change Your Skin In Batman: Arkham Knight

This guide will help teach you how to change your skin in Batman: Arkham Knight. You aren’t taught how to do this so it is possible you might miss it. In Batman Arkham Knight you can change Batman’s outfit and the Batmobile Skin.