I usually use the search function, but I couldn't find any guides regarding about this.
If you have good reasons, you can ask theymos. I don't think he accepts any username changes lately.
You can just create new account if you wish. Alt accounts aren't prohibited as long as you don't do something frowned upon by the community.
I'm not agree with it creating accounts, could led us to be red tagged. Most of users here only create new accounts when they're main account get a red tag, especially scammers.