Audi recommends using VAG-COM to retract the 12-volt step-motor on the rear parking brake before changing pads. According to the Audi enthusiasts, however, there are two alternative methods that don't require hooking-up to the car's computer:
1. With everything on the calipers still fully assembled, disconnect the electric connector from the caliper. Inside the connector on the caliper, you'll see two medal pins sticking up. Using a 12 volt battery source, connect a positive jumper to pin 2, and connect the negative jumper to pin 1. If the caliper piston starts to push out, stop and reverse the electrical leads. You should hear the parking brake retract. Initially, the piston will not move. Continue until the caliper piston starts to spin, then stop! Now push the caliper piston back into the caliper with a brake tool, channel locks, c-clamp, etc. Replace pads as you would normally. After all the pads have been replaced. Pump brake pedal until you have a firm pedal. Then turn on and off the electromechanical parking brake a few times and you should be good to go.
2. Release the parking brake, jack up the car, remove the wheel. Remove the two bolts holding the caliper on. Then wiggle the caliper free, remove the old brake pads and install the new ones. Then you can either use pliers, or or rent a piston retraction tool from Pep-Boys, to turn the piston in the direction which will wind-back the piston into the caliper. The tool, with various adapters, will look something like this one:
Amazon.com: Neiko 20733A Disc Brake Pad and Caliper Wind Back Tool Kit, 12-Piece: Home Improvement
You need to put on the correct adapter, then attach it to the tool, and start to wind back the piston. It will take a little while and it will sometimes not go at all. Just take the tool off and reposition the adapter and turn some more. You will start to notice after checking if it fits back over the new pads, that it is getting more clearance. Keep this up until it is fully retracted. And again, it may feel as if its not going back any, but it is, just keep spinning, and if it gets stuck, take off adapter and reposition and keep on spinning. If after you reposition the adapter and it doesnt spin anymore, check fitment and it will slide right on. After all the pads & calipers have been replaced. Pump brake pedal until you have a firm pedal. Then turn on and off the electromechanical parking brake a few times and you should be good to go.
I have not personally tried either of these methods, but can picture how each would work. If interested, you could always do some searches on the Audi forums for confirmation. Always wear your safety glasses when working on brakes. Some of those clips have a way of flying off in all directions. And always use jack stands to secure the car....don't rely on a hydraulic jack. Hope this helps. Best of luck.