Hi JohnJaguar. Welcome to the forum. If your parking brake is built into the caliper with a lever on the back of it, you don't have to release anything but you will notice two or four notches in the piston where it contacts the pad. A special tool is used to rotate the piston while applying light pressure to screw it in. That tool can be borrowed or rented from some auto parts stores.
Once the new pads are installed and the calipers are mounted in place, the parking brake must be cycled to work the pistons out until the pads contact the rotors. Unlike the front pistons, the rear ones will not extend by just pumping the brake pedal. If your parking brake cables are rusted tight and not working, you can use a large pliers to work the levers on the calipers to extend the pistons.
When you pump the brake pedal to adjust or test the brakes, never press the pedal more than half way to the floor. Debris and corrosion build up in the bottom halves of the bores in the master cylinder where the lip seals don't normally travel. Pressing the brake pedal all the way to the floor runs those seals over that junk and can rip them. That will result in a low and / or sinking brake pedal and will require replacement of the master cylinder.