When you are on the main command interface, tap the More (•••) button in the lower left area. After which, select Account from the available options.
Tap Change to show the currently available avatars. Remember, you'll will need to use a Change Avatar item. If you don't have any at the moment, you can always get one from the in-game store.
When you see something you like, just press on it to select. If you wish to get out of the avatar selection simply press anywhere outside of the avatar panel.