In the Xampp folder, there is a folder called FileZillaFTP. In that folder, there is a program called "FileZilla Server Interface.exe" (it may or may not show the ".exe" at the end).
Run that, and you will get the controls for the server. There is an "Edit" menu at the top, and you can find the Edit Users page there.
Come back and ask questions when you hit a wall on that. (It's not the most intuitive interface I've ever seen.)