If your field is a list box with 2 columns, I assume the data is pulled out of a table.
In this case, you need to update this table. Either through code, where you dynamically update your table, or with an update query that filters the data based on the selected value (preferably the primary key for your SeriesName). You end with requerying your products field.
Sorry I can't give you a more specific solution, You have to give us more detail about your form (type of field, underlying data, use of sub form, etc).
I assume you have a table, or query, resulting in something like this:
Dohicky One 12
Dohickey Two 4
Dohickey Three 78
I assume that this is the rowsource for some combobox, and when the user selects from that combobox, you want to deduct one from the QuantityOnHand column.
Assuming that the Product field is a uniquely indexed column, on the AfterUpdate event of the combobox, you can add an update query which will deduct one from the QuantityOnHand column. You can do this with a named querym or an SQL statement and if you've been following some of the other threads in this forum, you know which I prefer.
You will then need to cause a requery of the combobox's rowsource 'cause you want to update the QuantityOnHand column with the latest information. The requery is simply: