There are two forms of user names in OS X: Your long user name and your short user name. You can use your short name instead of your long one, if that's what you mean by your user name. Just use your short name once, and in the future administrator dialog boxes will show it instead of your long name. Or, if you don't like either name, you can create another administrator account with a different short and long name and use that instead. It's actually a better idea from a security standpoint to have your administrator account be different from your regularly used account.