You can currently change your @username both in-game and through rec.net.
Changing your @username in-game
You can change your username in-game by opening your watch, going to your profile, going to account settings, and setting a new username.
Please note that you can only change your username once every 180 days. We will not be able to circumvent this restriction.
Changing your @username through rec.net
To change your @username, please visit http://rec.net. Log into your account and navigate to your account settings by clicking the settings cog on the left.
Once you have done that, in this page you will be able to change your @username and nickname/display name. While you can change your nickname or the name that people see above your face up to 3 times a week, you can only change your @username every 180 days. Please choose wisely and make sure there are no unwanted characters or spelling mistakes.