Which side? If it's bore cap side then pop the bore cap off.
It it's not then its a bit trickier. I did it by pumping out the moving one, then adding more fluid, and pushing the moving one back in, the pressure pushes the stuck one out.
If it's really stuck then can you removing the bore cap side, then
*** maybe a stupid idea ***
drill it, tap it, put a bolt in and pull it out with the bolt. Mew pistons are quite cheap, and you are going to chuck it anyway. It might crack when you drill it and then you can remove the bits.
You can pull the old seals out with an o-ring pick, orbig needle.
Good luck (I found fitting new seals was really fiddly when I cracked a piston).