How can I replace the back cover of my iPhone 4s?

Hi,

The back cover of my iphone 4s is shattered. I need to replace it. I tried replacing it with a back cover that I got from Direct Fix, but it just didn't fit.

I have an unlocked iphone, does that make a difference? I was planning planning to buy a back cover for iphone 4s from ifixit and I was just wondering if I buy it for A1387 it would fit my iphone or not? Thanks.

Let me know in case any further information is required.