I see ... is there a problem with having
username=whatever in the URL? It would look a bit ugly, perhaps, but just look at the URL for this page.
An alternative would be to have a list of names, with buttons next to them - then you could execute multiple actions. ie., update, delete ... etc.
Yet another method would be to require a login, then create a simple link to edit their information (much like the one on the top of pages here) ... if the username, etc, are stored in session variables or cookies, then you won't have to worry about the URL.