sure, it could be after the line:
$eCart3->UpdateCart();
or it could be after this line:
$eCart3->cartAction = "Update";
you could even create a new code block to perform the update and use the trigger if statement around it:
<?php if (isset($_POST["eCart3_Update_100"]) || isset($_POST["eCart3_Update_100_x"])) {
<update code here>
}
?>
the point is to trigger the update when the update button is pressed.