The first time you log into the Community during the new user registration process, your Canvas username will be your default 'community username'.
If you would like to update your username, you have two options (at this time).
OPTION 1: Edit your username during the registration process
OPTION 2: Send a request for 'name change' to 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 help ticket so we may resolve the issue from our end. How do I file a ticket with Canvas Support about my access to the Community?
NOTE: Please note that 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. You just won't be able to delete or edit it).