The cistern or tank is the part of the toilet that sits above the bowl. The cistern pulls in water from your home's water supply and holds it until you press the flush handle. The water then rushes out of the cistern and into the toilet bowl, carrying away sewage into your home's sewer system. If your cistern becomes cracked and starts to leak, you'll need to replace it with a new one. The good news is that you don't need to call a plumber. Replacing a toilet cistern is fairly uncomplicated.
Turn off the water supply, which is usually located on a valve beneath the toilet. Turn it clockwise to shut off the water.
Flush the toilet to remove the water. Wipe up any excess with the sponge and towels.
Unscrew the water inlet hose from the bottom of the cistern.
Use the adjustable wrench to loosen the nuts that hold the cistern to the toilet bowl. Newer models may use plastic toggle nuts instead of metal ones. Remove the washers as well. Reach inside the cistern, and pull out the bolts and rubber washers.
Lift the cistern up, and set it aside. Replace with the new cistern. Make sure the cone gasket in the bottom of the cistern lines up with the hole at the back of the bowl.
Wrap the inlet on the cistern with plumber's tape. Attach the water inlet hose.
Turn on the water supply. Allow the cistern to fill. Flush the toilet, and check for leaks.