You should check the model number first if this is a dual sim version?
You can check the model number of the Sony Xperia by using this:
[image|777512]
You can buy the back cover from here: (It fits Sony Xperia Z5 E6603 E6653 E6633 E6683)
http://www.ebay.com.au/itm/Original-Glass-Battery-Back-Cover-door-Sony-Xperia-Z5-E6603-E6653-E6633-E6683-/262155023189?var=&hash=item3d09a83355:m:mv6HGRJhN6yYGcqcKUoKx0Q
Here's the guide on how to do it: Note that you only need to do step 1 and step 2.
[guide|52649]
Note: Please be careful because if I'm correct there may be a white NFC sticker tag on the back cover (It is a white sticker with two golden contact points). This model probably doesn't have it. If it does, you'll need to heat it up then carefully separate it from the back cover as it is held by adhesive.