Please send a request to support@hypothes.is with the username you want (along with a backup in case your desired username is taken).
After we change your username, it may take some time before your new username is displayed on all your annotations.
Note: Usernames must be at least 3 characters and no more than 30 characters. Accepted characters are letters a-z (lowercase and uppercase), numbers 0-9, periods (.), and underscores (_).