Raz
You can change your username by visiting this page: http://www.killermovies.com/forums/namechange.phpOnce you have submitted your request, it will be placed in a queue subject to moderation. Almost all requests will be approved, it is only there to stop abuse.
Please note that once you have changed usernames, you will lose your current one permanently. That means you will never be able to use it again in future. Only change names if you never want to use your current username every again.
If you have any problems/questions, please post them below.