Purchases made between October 29 through December 30 can be returned as late as January 14. The exceptions to this are cell phones, devices that can be activated, and AppleCare monthly plans, which all must be returned 14 days from the original purchase. Bring the original receipt, gift receipt, or packing slip to receive in-store credit.
Most items have a 90-day return window, so you'll be able to return the gift as your aunt wasn't doing her Christmas shopping in early fall. You won't necessarily need a receipt as long as the package is unopened and the product is undamaged. It is possible to be turned out down without a receipt though, so your best bet will be to show up with one if you can.
Generally, new and unopened items must be returned within 30 days of delivery, but during the holidays you'll automatically have until January 31. Just head to the Online Returns Center and select Return a Gift. Then search via your gift's Order ID, located on the left side of the packing slip. If you don't have a packing slip, contact Amazon and they may be able to locate the order in another way. You'll then receive instructions for how to ship the gift back. Once the gift is returned, a gift card will be added to the account you accessed the Online Returns Center with.