I think...click on your name (it should be blue and underlined) in one of your posts. You can view public profile. You might be able to change it there, but might have to type in your password to verify identity.
I don't think you can other than registering a new username and going by that one instead... but by doing that you lose your post history, pm's etc. I was thinking of doing this and switching to my more common screenname a few weeks ago, but decided to stick with this one
We don't advise you register twice- and if you do then one of your identities will be deleted. Generally the only time we allow members to change their username is when they use their full name without understanding the repercussions of this act.
You could wait until you have over 1000 posts (I can never remember if it is 1200 or 1500) and you can get the place where it says "Alpha cat" to have whatever you wish
Sam, she was referring to her actual username and not her usertitle. Speaking for the site, hissy's answer is the one which should be heeded here. We do not change usernames, except in extreme cases.