Users switching from one operating system (Windows, macOS, Chrome, Linux) to another often find that it's the simple things where they need help. Like changing your user information. This article provides the step-by-step for changing your password and basic user information.
Unfortunately, the dialog box displays both the term "Full Name" and the term "user name." These should not be confused with the term "username." What you are changing here is your full name that is used to identify you throughout Linux.
Changing your username, the login name that Linux knows you by when you log in and in the terminal, is not something you can do graphically. You will need to use a couple of commands in a terminal to accomplish this. This article on the Ask Ubuntu website explains the how and the why.
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