If you wish to change your username you'll first need to login to your profile.
1. Click on the menu icon on the upper right corner of the page (to the right of your picture).
2. Click on the 'Settings' section.
3. Under 'Account Settings' click 'change' to the right of 'Username' to update this information.
4. Type in the new username you wish to use. Click change username when you're done. Your username can not be in use by another member, or contain any spaces or special characters (#*&%!).