+1. Only the admin can change your username in Vbulliten forum.
Since you just joined and only have 5 posts I see no reason why he won’t change it for you. I misspelled my username when I joined a forum( may have been this one) and the admin corrected the spelling when I asked.
Just send a message to the admin.
[UPDATE]. I just noticed that your first few posts and replies are under a different name, so I guess it was change already! If so you may want to update the first post.