The back glass is attached to the back of the frame on the iphone 5, The only way to replace is to simply remove the glass with a heat gun and reapply with a new glass part. These parts are some what hard to find.
[http://www.ebay.com/itm/One-Pair-Black-Glass-For-iPhone-5-5G-Battery-Back-Cover-Camera-With-Free-Tool-/310505216075?pt=LH_DefaultDomain_0&hash=item484b8d984b|here] is one
Hope this helped.