It's a simple task!
1. Click on your user settings tab
2. In the My Account tab of the User Settings menu, click that edit:
3. Change that username! Think of a schnazzy new name and type it on in:
4. Show Discord you mean business by adding your password:
And you're all set! Keep in mind that usernames aren't unique, so you do technically have the ability to clone your friends' names. Remember, with great power comes great responsibility.. And it's probably best to not piss off your server mods.
Important note: Username changes are limited to 2 an hour.