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You have information stored in a database for each user of your site. When they login, you should have a similar form to the registration form that gets populated from the data you have for that specific user in your database so that they can make any necessary changes to their information. Upon submitting this form, a query will be sent to the database that will properly UPDATE the table with the new information.
First implement a form that grabs the data from the table and puts it into its respective input fields. Then, build the update query properly and you should be fine. There are a few examples of how this can be done floating around webmasterworld somewhere, so a little search might help you get started.
$query = "SELECT * FROM `table_name` WHERE `username` = '".mysql_real_escape_string($_SESSION['username'])."'";
I'm afraid you aren't going to be finding a ready-made script for something like this; you are going to have to do most of this coding, while not a lot, by yourself. Of course, if you need help along the way don't hesitate to ask, but I'd like you to kindly try to implement your own solution first and then we can work from there.