Click your name on the top right of the screen, then click account management. Scroll down until you see your current name and change it to whatever you want ( as long as the name follows basic rules and doesn't contain profanity , etc).


Note: you can only change your name twice every year (~168 day cool down in between name changes)

just click on that person's picture up there on the right, look down and then you will see "account managment" click it then there you go