The customer would have to select the order from their order history.  and then request a replacement or refund.

The customer would then need to select either replace and refund, the quantity from their order and the reason for return.

The return will then be available for review in the admin  Orders > Return requests

By selecting the request, the administrator can view the information entered by the customer and decide on the action.

This will then notify the customer and amend their order history to reflect the returned items.