Updating your Username
You should be able to edit your username in your Account Settings.
Letters, numbers, and the following characters are allowed in usernames: -._+=:;~!@$*(),<>
At this time, only members who used to display a username in the old site (rather than their real name) can continue to use the username on their profile. If you don't want to go by your full name on your profile, you can always adjust your privacy settings to just go by your first name.
Note: if you currently have your Username displaying on your profile, and you would like to switch to display your Full Name on your profile, click the 'display First name' option as shown above, Save changes, and then un-check the 'display first name' option.