You made a typo in your username, and now you want to change it. Good news - it's easy!
NOTE: Changing your username will break all social links that you have shared previously. Links do not redirect, once the username has been changed. Please keep this mind if you proceed to change your username.
Go to your Account Settings on the web (http://untappd.com/account/settings), and the click the "Change username" link on the first line:
Then change your username and click submit:
It's that simple!