Hi Vishnu,
You change the qty in the returns screen to 5 quantities out of the customer return qty of 10 nos for the process "with credit memo".
For the replacement or zero credit memo process, you create an Sales order with reference to the Customer Return ID. Now the sales order now picks the qty from the customer return id(say 10 nos), hence change the quantity manually to 5 nos.
This way you can do both the processes.
Regards,
Samraj