With your car on a solid level area, jack up the rear of the car and put jack stands in place. Then remove the rear wheels.
Using a C-clamp compress the brake caliper 1mm only. More then that and you risk damaging the parking brake. This is only needed to allow the old pads to slip off the rotors.
Now go behind the hub and locate the two bolts and remove them. Use caution around the wheel spin sensor.
Now slip the caliper assembly off the hub.
If your changing the rotors at the same time you may need to spray a little penetrating oil around the central hub and bang on the rotor with a rubber hammer. It should then come off.
Now simply slide your new rotor in to place. Nothing special to do this.
Now you will need something called a rear brake caliper turn in tool. You can find these and most auto supply stores and they don't cost very much.
Find the correct side for your brakes and turn the caliper clockwise to release the parking brake. You can then compress the piston.
Place your new pads in to the caliper.
Slide the caliper and pads over the rotors and line up. Then tighten the bolts.
Put the tires back on and lower the car. Start the car and leave it in park, PUMP THE BRAKES.
The job is now done.