How to Update Stock quantity if I sold items?

Hi all, please I want to update my Stock Quantity if I sold Item X or Y or Z and added the quantity of the item sold.

I attached my excel sheet, how could be a possible easy way to automatically update the stock quantity if i changed the sold items quantity?

I have A2:A6 as a Drop down List . Items are "Batteries" <== name list


P.S The Quantity of each battery name is under Stock beside the name of each battery.

Thank you!