Replace the Flapper
Replacing a flapper is easy. The first step is to drain the tank.
Turn off the water at the shutoff valve, flush the toilet and hold down the handle. There may be a little water left in the bottom of the tank, but don't worry about it.
Wipe the flapper seat with a clean cloth to make sure it's free of debris, and check it for cracks or splits (Image 1).
Check the flapper to make sure it's not torn and that it fits tightly against the seat. If the flapper is damaged or worn, pull it loose and replace it. Just pop a matching flapper onto the hinges at the base of the overflow tube (Image 2).