Assuming you have the standard drum brakes, you'd need to safely jack up and support the car on jack stands, remove the rear tires and brake drums. The drums are attached with two #3 Phillips screws which are not usually easy to get out so be sure you have a #3 Phillips screwdriver to remove them. You'll probably need a squirt of penetrating oil for the screws.
You'll see the brake adjuster once the drum is off. That's where you adjust the rear brakes. .