The piston doesn't screw in, that is usually for vehicles with rear disc brakes that incorporate the park brake in the caliper. The correct way for yours is to open the bleed screw and compress the piston. The most common (shade tree) way is to simply use a screwdriver and pry against the rotor pushing the piston back. Some pistons are ceramic so be careful not to crack it. If you pry between the old pad and rotor you should be ok.

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thanks...

Charlie