If your stock has declined and you want to update the stock count, this is what you do:
1. Scan the barcode of the product or start typing the name of the product. If you do the latter you should see the full product name followed by (already on list) which you can tap to open the product.
2. The product details screen, you can select an amount at the top and tap Consumed. This will decrease your stock registration by the amount you entered.
Alternatively, you scroll through the list until you see the item in question and swipe it to decrease the stock registration by 1.
Also note that if you choose to remove the item from the stock screen, this means that it's totally removed from the list and the barcode could also be removed from the entire system if nobody else is using it. This is handy if you made a bad product registration.