Record updates

Dear sir/madam
I wish to know how you can update a record in one table from values in
another table. E.g. I created a Purchase_order table and a
Purchase_order_detail table(from which I created a subform to the Purchase
form.) In the detail table I created a checkbox (PostedToInventory). From
there I wish to update the inventory table when the stock has arrived, i.e.
when the checkbox is clicked.

Furthermore how is this done with the default value "Purchased" in the
TransactionType field?

Thanks
Herman