If you have an older style American Standard toilet, it uses a circular rubber disk instead of the flapper valve found in most other toilets. Remove the valve by determining if it snaps off the plastic assembly or unscrews from it. After several years in the tank, the plastic becomes brittle, so work carefully to avoid breaking it. Replace the old rubber disk with a new one.
In most cases, it is best to turn off the water supply valve to the tank before replacing the flapper, although replacement only takes about a minute. Once the water is off, flush the toilet, and work with an empty tank. Once the new flapper is in place, turn on the valve again. Since the repair only takes about a minute, if necessary, you can make the replacement without turning off the valve.
The flapper valve seals the opening between the tank and bowl of the toilet. Regardless of its design, manufacturers make it float in the water once the user activates the handle and close again once the tank empties. With the valve closed, the fill valve continues to allow water in the tank until the float stops it. Minerals from hard water and automatic bowl cleaners that dispense in the tank speed the deterioration of the valve.