Do a search for calliper swap, that will give plenty of posts to look through giving the DIY
You will need to refill and than bleed the the line you are working on. If the fluid is old or you are unsure you should think about bleeding/filling the other callipers through the bleed nipples and the hydraulic clutch
Best way to get the calliper piston out is with a garage compressed air line after removing the brake hose use the hole for the nozzle. put a 1/2"thick pad of cloth jn the slot and cover the calliper with an old towel. The piston comes out with quite a pop and fluid sprays everywhere. check the old piston carefully to check it is OK to re-use. If seals are old and tatty or the fluid has never been changed corrosion inside the calliper can damage it and the piston