Hi Abhijit,
you won't be able to change the quantity on item level in a userexit or BAdI. Notice that in your picture the quantity field on the line item is greyed out which means that you cannot change it via manual input. But this also means that you cannot change the same field via a BAdI as the field selection is checked in the standard code that calls the BAdI.
Input fields are changed to display only for a reason, usually to maintain data integrity. In the case service PRs and POs all information is stored on service level not on item level. Furthermore, you could have several service lines with different units of measure that are of different dimensions. This would make it impossible to determine an overall quantity for the item.
Beate