Pull the rear pads out (remove pin and anti-rattle spring...NOTING carefully the springs asymmetric position), then, index (mark) rotors (probably LESS important on rears) and pull them off (makes it easier to resurface them). Now, rotate rear piston CLOCKWISE til it stops using correct spanner tool...OR CAREFULLY use needlenose pliers to engage the two slots on the piston to reseat...being CAREFUL NOT to slip and lacerate dust boot. If all goes well and you do not see any evidence of fluid leak, restall rotors to marked spots, place new pads into position, put anti-rattle spring in CORRECT location, and slide pin back in making SURE that you eagage outer and inner loops of spring with pin...tap pin into final position, and reinspect carefully to be SURE.
Call me if you need more...and, if I have forgotten something, hopefully, others will jump in and correct above.