I need to update a field for units in stock in a Products form after a customer orders a certain product(s) from an orders form, subtracting the quantity of the item ordered from the units in stock field for that item in the Products table so it is known how many of that item are available.

Use a query to return the units on hand base the form on the query, or put a calculated expression on your form, storing quantity on hand in a table is not recommended.