1.   Open the tank. Reach in and push down the flapper. If the water stops running, that means   that the flapper needs to be replaced.
2.   Shut off the water supply. Flush and drain the water in the tank.
3.   Remove the flapper.
4.   Take the old one to the hardware store to make sure you buy the right one.
5.   Follow the installation instructions.
6.   Adjust the chain so that the flapper lies flat, with just a little slack.

Note: If the water doesn’t stop running when you push down the flapper, it may mean that the water level in the tank is too high. The water level should be about ½ inch BELOW the plastic drain pipe.

If the running does not stop and all else fails – call a plumber.