Hello! To update your username, you'll need to go to "My Account" first - there is a button on the upper right side of your screen (on Desktop mode) right between "Save" and "Logout". On the "My Account" page, information about your account as well as your family trees is stored. The first thing under "Account Details" is "Username". If you click on "Edit", you'll be able to type in your desired username.
Hope this helps!
