Your cylinder is probably frozen. You will need 2 tools to remove the drum. An old hack saw blade and a brake adjusting spoon.
Put a bend in the last 1/2" of one end of the blade. You will use this to lift the adjusting arm off of the star wheel. Spray large amounts of WD40 or similar around the stud holes and theaxle end to help loosen the drum.
Remove the brake line, push the rubber boot off to the side, Looking inside of the drum thru the uncovered slot, locate the star wheel, Slip the blade inside the slot(on top of the adjuster wheel) and push the adjuster lever away from the star. Insert the brake spoon into the slot and rotate the star in the opposite direction of normal operation. This will back off the adjuster allowing removal of drum. If you are lucky you may be able to access the adjuster from the hole in the axle face side of the drum. You still will need to pry up the adjusting lever. Take a good look at your new cylinder and it will help you to visualize what you need to do. If you are real unlucky, you may have to take the drum off in pieces!
Good luck!
Mongo