Click on your name in the top right hand side of the screen, select My Profile, and once you do that you'll see an 'Edit My Profile' button click that. Once you're there you have a row of options to choose from on the left hand side of the screen. Choose the 'Display Name' option. After you click that, it will take you to a page to where you are permitted to change your name 3 times in a 30 period. If you have run out of chances to change your name, just wait a few weeks or so and you'll get 1 free new change every 14-30 days I believe. I hope this helped!
Colorful Respawn's post in How to Change Your Username...? was marked as the answer