Update count

Hey,

I'm trying to use the Fetch and Update records to update a count/total number of items.
eg I have an inventory form with toner cartridge with a count,
eg I have
Model: "lexmark T644"
Cartridge: "black"
number: "3"

And so on for each item.

In another form, when people input they changed one of these, how can I update the count from 3 to 2?
eg minus 1?