You might remember a while back when you picked your custom Facebook username. This username appears in your Facebook (vanity) URL, Facebook e-mail address, and it can even be used as your log in to save keystrokes. At the time, Facebook led you to believe that the username couldn't be changed once you picked it. Well, surprise, that wasn't actually the case.
It seems there is a one-time switch allowed, and as long as you haven't exercised this privilege yet, you're able to pick something new. If you never picked a username to begin with, now's your chance to secure a better URL and e-mail than the randomly generated number assigned by default. Remember, though, your old username/e-mail/URL will not work anymore once you pick the new one.
Step 1: Log in to Facebook from your favorite Web browser, then click the Account menu button in the top right-hand corner and choose Account Settings.
Step 2: In the new menu area that loads, click the Edit link on the same line as Username.
Step 3: Enter your new preferred username into the text box and then enter your current password so you can save.
Here are some guidelines that Facebook.com provides for picking a username:
You can't claim a username someone else is already using.
Your username should be as close as possible to your true name (ex: Jsmith or John.Smith).
Choose a username you'll be happy with for the long term. Usernames are not transferable and you can only change your username once.
Usernames can only contain alphanumeric characters (A-Z, 0-9) or a period (".").
Periods (".") don't count as a part of a username. For example, johnsmith55, john.smith55 and john.smith.55 are all considered the same username.
Usernames must be at least 5 characters long.
Your username must adhere to Facebook's Statement of Rights and Responsibilities.
Now you can update your social profiles on the Web with the new username so people can find you on Facebook.
Looking for more Facebook tips? Check out Sharon Vaknin's Seven hidden, superuseful Facebook features.
(Via Digital Inspiration)