You cannot, once a case is closed it cannot be reopened. For a returned item? Contact the seller tell the item is on its way give them the tracking number and hope they are honest and will refund you on receipt of their item.
If they do not, you would need to open a case with Paypal enter the tracking number, and escalate the case after 2 days and before 20 if not refunded beforehand.
Paypal here do take longer to refund, as they delve into cases more deeply, than eBay do.
