Once your Lagunita account has been created, you cannot change your username. In addition, no other person can have the same username since all usernames are unique.
Your unique username displays on your dashboard, in exported grade reports that the instructor runs, and in the discussion forums such as the example below.
If you need to change your username for any reason, you can contact the platform support team by clicking on the Help link that displays on the top right side of Lagunita pages. The Help link takes you to the Help Center which is where you can submit a request to us. You can also submit a request by going to https://stanfordonline.zendesk.com/hc/en-us/requests/new.