I believe that you cannot do this :)
It is populated during authentication. The simplest solution - when the user changes the username - log out and ask to log in.
Update
It will only work with a custom provider. If you use the standard, I believe that it cannot be changed at all.
As an alternative solution, you can try to do the following: When a user tries to change his name: 1. Create a new user 2. Cancel the session 3. Delete the old user 4. Change all the data associated with the user to a new account 5. Write it down again with a new user.
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