<?
require($_SERVER["DOCUMENT_ROOT"] . "/bitrix/modules/main/include/prolog_before.php");
Cmodule::IncludeModule('catalog');
$iOstatok = $_GET["QUANTITY"];
$iElementID = $_GET["ID"];
$el = new CIBlockElement;
$arFields = array("QUANTITY" => $iOstatok);
CCatalogProduct::Update($iElementID, $arFields);
?>