Update query using two tables
You must specify the condition in a new column where you select the OrderID column.
So, in the 3rd column select Table tblOrderProduct and Field OrderID and place [Enter Order ID] on the Criteria line of this column.
Replace the * by -. This will subtract the Quantity from the ProductStock.
Well, and you don't need the Quantity column. Remove it.
In the SQL view you will see
UPDATE tblProduct
INNER JOIN tblOrderProduct ON tblProduct.ProductID = tblOrderProduct.ProductID
SET tblProduct.ProductStock = [ProductStock]-[Quantity]
WHERE (((tblOrderProduct.OrderID)=[Enter Order ID]));