While there are some scenarios in which the buyer or seller can cancel an order, it's not possible to process a refund & return for a shipped order. That's because the shipping label the buyer paid for during checkout has already been used and is therefore no longer refundable.
For the rare occurrence in which the seller accidentally shipped the wrong item, it's the seller's responsibility to make it right. That usually means shipping the correct item (and a return label for the incorrect item) on their own dime. If you've received the wrong item, please contact the seller directly from the order details page. Don't forget to rate the seller's performance once the issue has been resolved!