HOW CAN I CHANGE MY USERNAME?

[Kaysi]

how can i change my username?

[Rolig Loon]

You can't.  Ever.  It's like your RL Social Security number or a really bad tattoo.  It's yours for life.  You can either open a new account (an alt) and choose a different username there, or you can create a Display Name that many people will see in world in addition to (or instead of) your username.  It's like a nickname.  Just like in RL, you can't use a nickname on anything official but your friends and family will know that's what you want to be called.  Read more at http://community.secondlife.com/t5/English-Knowledge-Base/Display-names/ta-p/700173

[Ansariel Hiller]

Like Rolig said: Username can only be changed by creating a new account. I personally wouldn't rely on display names. They are borked and often turn out to not work properly as well as many people don't even see them.