If you are an admin, go to the AdminCP > Users Management, find our your username and edit it. If you are just a member, you can't do that.
if you are a member then you may not have changing user name choice on your control panel options. you need to request administrator for the change. if it is a small forum & if you have less number of posts then it would be easy & admin might oblige your request. if its a large forum then he might decline..
Admin can do it.nither a user nor any moderator can do it.there is a option in admin cp,by which you can do it easily.
Only admin can change your username. So contact with forum admin, and tell them why you want to change it. Hope they will help you