Change in service order line item qty

If I have created one service order with one line item which have 3 component in services tab, if partial services availed then after if I have to reduce the qty of particular components then pls provide steps to make amendments

Example:

Line item 10: construction work qty 1 value 100000

Service component :1) labour = qty 4, rate 5000=20000

2)supervisor = qty 2, rate 20000=40000

3)PCC work = qty 10, rate 4000=40000

If service of 5 qty for PCC work is done and also posted the service entry for the same, and now if I have to lesser down the qty to 7 due to lesser requirements for PCC work, but while doing the same system is showing error of GRN is exist you can't change the order

Pls suggest