What jbone meant to say was...."Simple: You don't change it
But I will actually answer this.
Phade will not allow anyone to change their username so that no one posses as another creator and causes mayham. You have to write an e-mail to Phade and explain to him why you want it changed. It better be good to because he usually doesn't change it otherwise.
But I would wait till the site is up to maximum workage order stuff, then he'll have more time for personal requests.