If you are wanting to colaspe the piston so you can fit new pads in the caliper, first open the master cylinder, then use a "c" clamp or "c" clamp vise grips to slowly push the piston back enough to...
If the emergency brake is not connected to the caliper use a "c" Clamp
If the emergency brake is connected to that caliper a special tool (not expensive available at Sears or auto parts) is needed...