See the Help Tab in the menu at the top of your screen...
Frequently Asked Questions About my account
4. How do I change my username?
We take requests for minor cosmetic changes to usernames of Premium members. We also take requests for username changes of any member if their username contains personally identifiable information. Because player reputation is important to the integrity of the game, we do not allow any other changes to usernames. Please don't create a second account as that will just get you banned. Open a support ticket below to request a username change.