It can't be done , you need to create a new account.....Sorry.
http://www.tomshardware.com/answers/id-2837545/admins-username-change.html
Two years later and i assume it still hasnt changed? We are still slaves to our old account names that we created 1, 2, X years ago and in order to change it we have to create a new account with what all our achievements and badges are gone?