Unfortunately you can't -- because the username is used as the primary id in the database.
The best you can do is to create a new user (and then set the status of the first one to drop). That will insure that only the new username is in use and that any work with the old username stays around -- but you will have to combine the grades from the "two users" by hand.
WeBWorK Main Forum
Can you change a student's username?
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