Change the quantity of the drop shipment SO

The senario is follows: I had create a sale order and a purchase order. They are linked drop shipment. Now my customer wants to change the order quantity, for example, from 10 pieces to 8 pieces. So I think I need to change the related purchase order. I did the testing but it is not successful. Can anyone suggest a solution? Thank you

Hi, You delete the Purchase Order Lines and again copy the Lines from Function->Drop Shipment-> Get Sales Order.It will take the updated quantity.