To help you understand the system, here's a description.
You have rear disc brakes, within the rear rotor's center hub there's a small drum and a set of brake shoes that are used for the parking brake. It's basically a small version of drum brakes. So if you've replaced drum brakes before, you'll have a basic idea of how they're adjusted. (You can do it while you're inside or through the hole in the rotor)
If I were you, I'd pull the rotors off and have a look at the shoes to check the pad thickness. If there's still quite a bit of pad left, then adjust them. If not, then replace the shoes then adjust them.
Once that's all done, if they still need to be adjusted, you can follow the cables under the car until the " Y" meets toward the center of the car and you should see an adjusting nut there.