You can use air pressure into the brake hose tapped hole, or since the boot is already torn, pry it out with a pair of screwdrivers.
You cannot twist it out and I hope you don't put pliers on the sides of the piston.
CAUTION: The piston is held in the caliper with a relatively large snap ring. When it breaks free, if you are not prepared, something will go flying. If you use air pressure, I suggest that you put a 1 cm thick piece of wood above the piston, below the two "fingers" of the casting so that the piston can only rise about 3 cm. At that point, it will have come free of the snap ring.
Second, can you then re-assemble it? There is another snap ring and several ball bearings that must also be removed, placed back into the piston, and only then reassembled. If done improperly, the hydraulic brake action will work (your foot pedal) but the ratcheting feature of the parking brake will not.
This assembly will still be in the caliper when the piston comes out.
To remove the assembly, look for this snap ring in the bore. You will have to pry the tabs inward to unhook it from the groove.
The Picture above and below is from someone that unscrewed the parking brake lever bolt arm without holding the lever arm. There is nothing inside that caliper that withstands the force, so things can break. PLEASE hold onto the lever arm when unscrewing the bolt.
There is also a set screw deep in the post where the brake hose bracket bolt screws in. You will need a 4.5mm allen head for this. You'll have to remove it to get the bearing assembly shown on the bottom of the right hand image I have above.