Issues with brakes while doing maintenance

[denhamfree]

Hi, I own a 2013 Mazda CX-5 Sport FWD Manual transmission. While trying to replace the hub assembly on the rear tire, I ran into the issue that the brakes were engaged even though the car was off and the hand brake was disengaged. Now I do not know how to turn off the brakes so I can do maintenance work on it. The piston is still engaged, and I can't squeeze the caliper back onto the rotor. I am pretty new to this kind of thing, so I apologize if I didn't explain it well.