Suggetions:
Using the sale order amendments can change the quantity and rates in locked order.
Menu :
SCM>>Sales>>Sale orders> Sale order Amendment
Step1:Go to sale order amendment and mention the sale order reference for which amendments require
Step 2: In the detail screen, change the item details,quantity,rate,discount% etc as per the requirement and click on finish.
Step 3: Confirm/lock the SO amendment .
Step 4: On the confirmation system add the change details in sale order.
Refere Screenshots for more details