Does anyone how to remove or decrease Material Request Item row from Purchase Order?

When the document is submitted and you are unable to change quantity of the items both material request and purchase order. In my case when the supplier delivered 70% from Purchase Order the quantity should be decrease to 30% as well as in material request because if you don’t update the quantity of material request it will remain partially ordered. Could someone help me how to remove item and update item qty in Purchase Order and Material Request? Thank you