Just ask Administration or Moderation team for access to change your username, or tell which usernames you wont and they will change it for you
We don't change usernames that have been multiply quoted(mentioned) in the forum ..... if your account doesn't have many quotes. you can ask for access now. do you wish for it to opened for you? We don't wont members to abuse it that why its locked
On top of the page is a menu bar, just click profile ,there you can change your user name by deleting the one over there and replacing it with you new user name, make sure to scroll down and click the save button. The profile menu also contains bunch of stuffs like profile picture(avatar), signature, friend request and others..