forms in access

i have 2 forms, first one asks for details of the printer and toner requested and amount, the second is a list of printers and tonners and amount held on stock.. how do i get the first form to reduce the stock amount held in the second form
i know i need to get it to identify the printer, toner colour in the second form and then reduce amount by what has been ordered from the first form
but i dont know how to do it.. pointers or help would be appreciated..

steve