After you remove the caliper, you can try to push the piston back in using a large C clamp, or even a vise, if you have one that will fit the spaces involved.
You can also go the other way. There is a tool that grips the inner diameter of the piston. The tool can then be tightened to provide more grip, and then with a twisting motion, the piston can be removed. You will probably find rust pits all around the piston. For a shorterm fix, you can polish/buff the piston and reinstall with a new seal. The better solution, once calipers become stuck, is to replace the caliper with a rebuilt one.