Shop stock control idea

Question

Hi, I am currently doin a stock control of a kind of cloth shop. Can you help me how can I reduce the quantity of the item by 1 when I press buy.
I need it to work like this select row from datagridview and press buy and quantity of that item is reduced by 1.
Someone help please thanks

Here you can find a screenshot of what i am talking about thanks alot. -- > http://imageshack.us/photo/my-images/23/buyje.jpg/