There are two main ways skins are installed in GetAmped. The method depends on the type of file you downloaded.
Place the .skin file into the \GetAmped\Skin\ or \UserSkin\ folder. Then select it in-game via "Custom Skin" menu.
Because the game uses a relatively simple cel-shaded aesthetic, changing the skin effectively changes the entire vibe of your character. You can turn a basic brawler into:
Detailed for creating your own HD textures. GetAmped 2 - Recolouring Skins