Okay- More detail? Use either hydraulic pressure (or better yet, air) to blow (gently) the piston out of the bore. Clean up, replace seal, lube with brake fluid, reassemble, bleed brakes.
Tip: After removing the caliper and brake line, place a folded towel or shop rag between the piston and caliper. Use a blow gun from your compressor where the brake line went, and in very short bursts, blow the piston out. The rag will prevent damage from the piston smacking against the far side of the caliper.
Whatever- the principle is till the same- use some sort of padding to keep the piston from flying uncontrollably and possibly striking something, and use compressed air to move it out.