How to fix the corners of my iphone 4?

I bought it used and I notices that the metal part is like shrewd off and the the very corners are damaged and on the receipt it says I could exchange it within a month I still have 3 more days till its the month but they said that's my fault I bought it used what should I say so they could give me another iPhone 4 Thanks.

Well, you can say that the boot does intermittently restarts by itself and also the battery do get real hot sometimes. Maybe it could be a valid reason to get you a free exchange.