you can't as far as I know, you simply have to create a new one if you want to not use your Sir Wynn user name. One reason I personally suggest not getting character user names.
I do not know if the back end of the forum has any admin tools that lets u do that, but technically if this is a standard phpbb forum someone somewhere has access to the mysql db that houses all the user names. If it was manually altered in the DB it could be changed. I assume the user is referenced by a unique ID as most dbs are set up that way so changing the username shoudlnt have any repercussions (Iin theory). NOW this is all assuming that someone has access to the DB and is willing to do the manual change. I would suggest emailing the board admin, to see if they can do it. Otherwise Dave pretty much laid it out.
My D&D forums use the exact same software. Jeremy is correct -- it can be done from the Admin tools if the owner (probably Mike V.) is willing, but not manually by a user.