A return means two things need to happen: the copy goes back into stock, and the recorded sale is corrected. The right way depends on how the item was sold.
Sold through a website order
Handle it on the order itself: open the order under Sales & Customers › Website Orders and cancel or refund it there. When you cancel, BookRouter offers to reverse the card payment and return the items to stock in one step. See Managing orders.
Sold at the Point of Sale or on an invoice
Open View Sales, find the receipt or invoice, and click Refund on its row. Tick Return items to stock so the copy goes back on sale. A full refund gives the money back (to the card, when it was charged through your gateway), marks the sale Refunded and takes it out of your sales reports. A receipt cannot be deleted; refunding it is how it is undone.
An invoice that has not been paid in full is cancelled instead of refunded: Cancel invoice returns any payment already taken and, with the box ticked, the items to stock. See Undoing a sale.
With Braintree or Authorize.net, a card sale refunded on the same day is voided rather than refunded (the customer is never charged), and a partial refund has to wait until the payment settles, usually the next day.
Sold manually (the Sell Item button)
Open the item and go to its Sales tab. Delete the sale line and choose to restore the quantity. The stock comes back, the sale disappears from your reports, and the status returns to For Sale if the item had sold out. You can do the same from the line on All Sales.
Marketplace sales
Process the refund on the marketplace itself (Amazon, eBay and the others handle their own payments). Then bring the copy back into stock by raising the item's quantity and, if needed, setting its status back to For Sale.
Why not just bump the quantity? Raising the number alone puts the copy back on the shelf, but the original sale stays in your sales history, so your revenue and margin reports would still count it. Correcting the sale where it was recorded keeps the books honest.