on 04-19-2013 03:16 PM
You cannot change your username. Ever. It's with you for life, just like your Social Security Number in RL. You have only two choices :
1. Open a new account and choose a better username for your alt. Or,
2. Use a Display Name. Many people will not see your Display Name or will ignore it, but at least your friends will probably know you by that name instead of your username.
on 04-19-2013 04:19 PM
Your username is permanent. This is to prevent scammers and griefers from abusing other residents, then just changing their names so they can't be identified.
If you don't have much invested in this account, just abandon it and open another, choosing a username you can live with. Otherwise, you can use display names, but as Rolig says, many people don't see or use them, so you'll still frequently be called by your username.
I've noticed this topic popping up a lot lately, presumably from people who are more accustomed to sites where your username is just your login name and you choose another name for others to call you. That's not the case with SL.
In the "old days," when residents still had first and last names, this was more obvious. Your username is also called your avatar name; except unlike in RL, you can't file for a legal name change!