After you reach a certain number of posts, you gain the ability to change the "Custom User Text" under your handle. I'm not sure, but I think its 25 or 50 posts when you go from being a "Junior Member" to being a "Member". After that you can change it under the "Edit Profile" tab in your User CP (Control Panel)
Once your account is created, it is not possible, to my knowledge, to change your handle.
Hope that helps.
SWG