The pistons screw back in.
You can do it with a pair of needle nose pliers.
As you look at the piston, you will see that there are 2 'V' shaped notches opposite each other on the face of the piston.
(I think this is correct, haven't replaced the rears in awhile)
You can grab the V notches with the pliers, and screw the piston back in clockwise.
Or, you can buy a tool for this at Sears, the "Lisle Disc Brake Piston Tool"
Be sure to line up the piston so that the actuator for the handbrake still works.
Can't explain that part too well, just look at it to see how it works.