You need a tool to communicate with the EBCM (electric brake control module). Then you can select retract rear pistons and it will retract them for you. This is the new big thing with the electronic parking brake. You can buy a special tool just for your car, or maybe a loan-a-tool from a parts house near you.
Edit: not sure if your model had it yet. I can't log in alldata from home. If yours DOESN'T have electrical wires going to the caliper, not the pad sensor or the abs sensor, but the rear of the piston housing you can definitely go to a parts house and borrow a screw in piston Kit from them.