When I first joined the forum I had to ask Josh, the forum owner, to fix my username for me. I just now went to the User CP function, and I don't see where you can do it yourself. Just send Josh a PM and tell him what you want to change it to, and he'll do it for you. You can find Josh here:
I just sent Josh a PM regarding user name changes and how he wants them handled. When he replies I will let you know. Ryanseiler sorry this has taken so long to resolve for you.