If your toilet is leaking or constantly running, it could be the result of a damaged flush valve or mechanism. Fortunately, this is a relatively easy part to replace, and you can have your fixture back in working order quickly. Drain and remove the tank, then replace the valve before returning the tank to position for testing./
Shut off the water to the toilet, and flush it to drain the tank. Place a bowl underneath the water intake to catch any remaining water in the bottom of the tank, and unscrew the intake hose from the toilet.
Unscrew the bolts that hold the tank onto the toilet. If they're rusted or frozen, cut them off with a hacksaw and replace them with new hardware.
Unscrew the spud nut holding the old valve in place, and remove the entire mechanism. Slide the new flush valve mechanism into place using a cone washer over the tailpiece. Align the flush valve according to the instructions provided with your hardware, keeping the overflow pipe facing the fill valve. After replacing the spud nut to hold the new valve in position, place the spud washer over the tailpiece.
Set the tank back on the toilet, and use the bolts to reattach it firmly. Replace the flapper, and replace the refill tube so it points into the overflow tube. Reconnect the water feed, turn it on and check to make sure there are no leaks.