How to enable and manage merchandise returns in PrestaShop
To enable merchandise returns, log in to the backend of your PrestaShop store, click on the Orders tab and then on the Merchandise Returns sub-tab. On the page that opens find the option Enable returns. It should be set to No by default, so just mark the Yes button.
After the option is enabled customers can generate RMA (Return Merchandise Authorization) slips. To do this they have to log in to their account on the frontend of your store, go to the order history, view the details for the particular order, mark the product(s) they want to return, and finally click on the Make a RMA slip button. The return status will be set to Waiting for confirmation by default.
You can check for return request from the Merchandise Returns sub-tab of your store's admin panel. At the top of the page there's a table with all the merchandise returns. Click on the name to view the details and change the return status to Waiting for package. This will generate a return slip that the customer can view and download from their account.
After the customer sends back the product(s) and you receive it, change the return status to Package received. You also have to create a credit slip. To do this go to the Orders tab, find the particular order, click on its name to view the details; on the details page mark the checkbox for the returned product(s), then mark the checkbox for Generate a credit slip (and optionally some of the other checkboxes), and click on the Return products button. You can view and download all credit slips from the Credit Slips sub-tab of the Orders tab. The customer can view their credit slips from the respective section in their account.
For screenshots and some more details you can also read the tutorial on enabling and managing merchandise returns in PrestaShop.