Is the order FBA or FBM?

For FBM order, you can check:

Orders --> Seller Fulfilled Returns

If you used Easy Ship, Amazon will automatically refund the customer when they return the item, and your account will be automatically adjusted. The item will be returned back to you automatically through Easy Ship.

If you self-ship, then you need to arrange pick up of the item from the customer and manually provide the refund in the Seller Fulfilled Returns section.

For FBA order - you don't need to do anything. Amazon will pick up the return and refund the customer and adjust your account accordingly.