Tap your name at the top of the main screen, then tap the second tab to open Player Profile. From there, tap your current avatar to see all available avatars.
To change your avatar, tap the avatar you want to use, then tap Show.
If you have your Facebook account linked, you can set your in-game avatar to your Facebook avatar by tapping Restore Default.
You can also set or change a frame on your avatar on the Avatar Frame tab. Tap the frame you want to use, then tap Show.