Assuming it is the caliper, it depends on if its the caliper slides or the piston.
If it's the piston, get a new caliper.
If it's the slide pins, get a new caliper.
If it's the pins, you can pull them apart, clean them up and clean the cylinder that the pins slide in with penetrating oil. BUT, the rubber boots that seal the pins in place may be torn, which let in water, which seized the pins.
You probably need a boot kit. Get it at NAPA for like $13. Use synthetic caliper grease. Get that at NAPA too for $4. When I do a brake job, I pull the pins and squirt some of that grease on them as a preventative measure.
If it's the caliper piston, any thing you do is just putting a big ole band-aid on it. Buy a caliper.