On the FRONT calipers, you just have to push the piston back in. There's no build-up of pressure. As long as nobody's stepping on the brake pedal, the fluid just pushes back into the reservoir. Make sure it doesn't overflow & make a mess.
On the REAR calipers, the handbrake mechanism prevents the piston from simply pushing in. So you have to turn it, like you're screwing it in. Be careful you don't tear the rubber dust-boot around the piston.
Front and rear, be certain you don't let the caliper hang by its hose. The weight of the caliper can damage the hose, then someday it'll burst. That's not good.
You don't have to bleed them if you just change pads. But a lot of people use the opportunity to flush fresh fluid thru the system by bleeding it. You should do that at least every 2 years, anyway. Brake fluid absorbs moisture from the air & that makes it corrosive.