The store I bought it from won’t exchange Apple Watch

I bought an Apple Watch for my fiancée and turns out she went and spoiled herself before I had the chance to give it to her for Christmas. I bought it Dec 23. I went back to the store I bought it from and they won’t exchange it. It’s still in the box and never been open. It’s a 38mm and I would like to exchange it for a 44mm for me. It’s an Apple 3 series GPS and cellular. I would be happy to pay the difference if you guys can help me exchange it at an Apple store. Still have the receipt as well if you need it.

[Re-Titled by Moderator]