You have to close the connection between Facebook and this forum first, or it will not let me change your name. You can do that in the "Edit connection" bit of the Settings page.
There should be an option in your UserCP (Click the 'Settings' button at the top tab menu thingy) to change your username, but I don't know if there's a period of time a user has to be registered before it shows up.
Anyways, for any further help on it, you will want to talk to FlashD. He will be able to help you change your username if you run into any problems such as the feature being disabled or the username already being in use. Hope this helps. :]
Great Witch of Britannia
AF's Annoying Weeaboo