Log into your profile on the site, and follow these steps!
1. Select the menu icon on the top right of the page (to the right of your picture), and under the menu that appears choose Settings, then Account Settings:
2. Under Account Settings, select 'change' next to Username to update this information. Your username can not be in use by another member, or contain any spaces.
3. Type in the new username you wish to use. Select change username when you're done.