Sandeep,
Depends on how your Product Allocation is set up. The most common method is to update the PA Planning area using a Planning book, and then to copy the updated qtys to the Product Allocation Group.
In any case, I suggest you refer to SAP help in this matter, where the entire process is described in detail. Start at the top of the left hand pane, and work your way down topic by topic.
Product Allocations Settings - Product Allocation - SAP Library
Here is the detail about copying to the PAG.
Copying Product Allocation Quantities from the Planning Area - Product Allocation - SAP Library
Best Regards,
DB49