If you try to compress it, you are likely to damage the calipers.  You need
to screw (rotate) them back in.  Use the grooves in the piston to rotate it.
There is a special tool that will allow you to rotate the rear calipers
which is useful but not needed.  Try needle nose pliers.  If they don't
budge, it's possible you damaged the caliper and need to replace it.

Open the bleed screw. Use a piece of Tygon hose to collect the bleed. The
small amount you lose should not be a concern. Top up at the master cylinder
if necessary.