update_product.php is handling the newQty variable... when u put in the new quantity into the input field u need a submit button for the field (or even set up one of those fancy hidden submit buttons where u just hit enter
seriously, for this.. if you're just trying to add a variable to the link while on that site, without refreshing.. look into javascript.. im not sure how it can be done, but im sure it can.
all u need to know is $_GET['newQty'];
that will grab newQty from the URL ..... <form method="get" action=""> (with the newQty input field inside it ) </form> will send newQty=(whatever they put) into the url that u are at.....
like.. im still confused at what the actual question is. lol