You would have to do this manually like:
<?php
$CustomQuery = new WA_MySQLi_Query($PowerCMSConnection_i);
$CustomQuery->Statement = "UPDATE tblitemculford SET ItemQty = ItemQty - ? WHERE ItemID = ?";
$CustomQuery->bindParam("d",$inthecartqty,0);
$CustomQuery->bindParam("d",$inthecart,0);
$CustomQuery->execute();
?>