If you are changing the front brakes, a large C-clamp will compress the caliper piston and reseat it. If it wont reseat, open bleeder screw when compressing with C-clamp. If it doesnt reseat, caliper piston is frozen and needs new caliper.
If you are replacing the rear pads, you will need a special caliper tool(can be purchased or rented from Autozone or Discount Auto Parts) to reseat the piston as it has to be turned or rotated inward to reseat:
NOTE: To move the piston back into the caliper, the LH piston is turned clockwise and the RH piston is turned counterclockwise. NOTE: A moderate to heavy force toward the caliper piston must be applied. If sufficient force is not applied, the internal park brake mechanism clutch cone will not engage and the piston will not compress. Using the special tool (or equivalent such as OTC tool 7317A), compress the brake caliper piston into its cylinder.
In an emergency, you can use needle nose pliers, but be careful not to pinch the rubber boot or seal as this will create a leak of brake fluid: