How do I return my replacement AirPod Pro?

Did you have to purchase this replacement pair or were they provided to you under a AppleCare+ lost item claim?

For the former, you have 14 days, from the date of purchase, to return them for a full refund. For the latter, you will need to contact Apple Support directly for your available options. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.

Sorry, but I don't actually know Apple's policy on returning an unused replacement item. Again, your best bet would be to contact Apple directly for that answer.