The caliper is what holds the brake pads. Some have two bolts holding it and some have one bolt and a pin. Take the bolt or bolts out and pry the caliper up off of the rotor. If your question is really how do you push the piston in on the caliper, if it is a hollow cup, use a C clamp to compress it. Lay one of the old pads across the face to push against. If the piston is smooth on the surface with a line across it or some figure like an hourglass on the face, it screws in clockwise.
Corroded calibers that are rusting and sticking. Moisture is in the brake fluid. Replace the calibers and replace all the brake fluid. It can also be a sticking slide pin on the brake assembly. Check that before replacing the caliber.
On the front calibers they should move in when compressed, the back ones have to be screwed in.
If you change the brake fluid every 50,000 miles the calibers will last the life of the vehicle.
Front brakes are done the usual way but the back pistons have to be screwed into the calibers not pushed in.
You have to remove the caliper completely except the brake lines. Then the pads will slide in a groove on each side of your rotor. Then you should be able to reinstall your calipers and that should take care of it.
It helps re-fit to slide it back a fraction.
If u are talking brake rotors remove the calibers and the calibers support the rotor should almost fall off in your hand
On the back calibers you have to screw the pistons in they don't compress in, you can ruin them if you try. They sell the square tool for a couple of bucks.
Calibers are sticking. You will need to replace the calibers that are sticking and flush all the old brake fluid out of the system then replace it with fresh fluid.
Brake caliper needs grease in grooves so it can slide out when brake is released and can slide back in when brakes are applied. Or it could be that the brake pad is broken,you have to get someone that knows what they are doing to pull the wheel off and check it soon or it may heat up the rotor to the point it will crack.
Brake calibers can be rebuilt, but my advice is to just replace it with a new one. Be aware that if one is sticking then more than likely your brake fluid has become contaminate with moisture. I would replace both calibers and not just one. Remove all the old brake fluid and install fresh fluid from an unopened can.
Rusty and dirty calibers, rusty mounting points, rusty caliber slide pins, or contaminated brake fluid. Brake fluid absorbs moisture and I would bet yours is severely contaminated. Completely flush your entire brake system and replace with the proper fluid from and unopened can. Bleed the brake system and see what you have at that time. Replace the guide pins if they are rusty or pitted. Lubricate them with silicone brake lube. Clean off all mounting points with a wire brush, do not use a grinder. You may need to replace the calibers to completely solve the problem. Brake fluid should be changed every 36,000 miles at a minimum.