Where Can I Exchange My Iphone?

This is a walmart purchase. I called to see if I could exchange it at walmart. I was told yes but they have limited quantity and I have to return this phone by monday. So my question is, if my local walmart/s don't have any iphones in stock, can I return my iphone to an apple store, or directly to apple via shipping for a straight exchange? (same color and amt. of memory).

I can't take care of the exchange until sunday, so the walmarts locally may not have stock.

I have had cosmetic issues with the phone very early on with a black iphone, and am going to ask if I can exchange it for a white iphone 5.

If anyone has been able to exchange at an apple store or directly through apple's HQ, let me know. This is my first iphone purchase and it seems like store policies on returns and exchanges vary from vendor to vendor.