How to pass data from form without a form field? (PHP)

I believe you are looking for

 <input type='hidden' name='username' value='theusername' />

hidden - can only be seen in the source of your HTML document
name - where it will be in the $_REQUEST/$_POST/$_GET ($_POST or $_GET depending on how you are submitting your form) variable on submit
value - the username you want this form to relate to

PRO TIP: Have a way to tell who is trying to update users so you don't have unauthorized people updating your user information. It would be very easy for someone to change the username in the form and try to update someone else.

You can use input type hidden

<input type="hidden" name = "username" value="<?php echo $username ?>">

use an:

 <input type="hidden" />

HIDDEN is a TYPE attribute value to the INPUT element for FORMs. It indicates a form field that does not appear visibly in the document and that the user does not interact with. It can be used to transmit state information about the client or server. Hidden fields often store a default value (e.g.via php), or have their value changed by a JavaScript.

more here