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.
Remember, you'll will need to use a Change Name item. If you don't have any at the moment, you can always get one from the in-game store.
After you enter your new name, simply tap Apply to confirm the change. Accepted names can contain only Numbers and Letters and it's length must be between 3 and 16 characters.