During the new user registration process when you first log into the Community, your Canvas username will be your default community username.
Currently, if you would like to update your username, you have two options.
Option 1: During Registration Process
When you create an account, edit your username. Otherwise your username will be your Canvas username.
Option 2: Send a Request to Canvas Support
It's possible that you may want to change the username after you have already registered. At this time we are looking into ways to allow the user to do this on his/her own. Until then, please create a 'Community Username Change' help ticket so we may resolve the issue from our end.
Note: Sometimes changing your username may result in the links to any content that you created previously in the community breaking. The content will still be there, however, you won't be able to delete or edit it.