Replacing a flapper valve is inexpensive and very easy to do.
First examine the flapper where it attaches to the standpipe. There are basically two types of mounting systems. The most common type has two little arms that project out from the sides of the standpipe near the base and the flapper has to rubber ring tabs that snap on the arms. They are very easy to change because the height of the arms is preset so you usually won't have any problem with needing to adjust the flapper.
The second type of mount is a ring type but they are not common. The ring type is a little bit more difficult because you have to adjust the ring so that the flapper is in proper alignment with the center of the flush valve seat and the height needs to be correct to prevent the flapper from leaking. I am including an illustratoin that shows the proper alignment of the ring type flapper.
Korky and Fluidmaster both make high quality flappers which are relatively cheap, typically about $2 or less.
They have now started making flappers that are sized to the flush rate of your toilet, so you should know what your flush volume is when purchasing a flapper. The GPF or Gal/flush should be marked on the inside of the tank. Very Old toilets are typically 5gal/flush. followed by 3.5gal/fl and the new water savers are 1.6gal/fl.
Fluidmaster now makes a universal flapper that has a rigid plastic mounting frame and the rubber flapper ball can be rotated to adjust for the differing rates of flush. They are about $5 but they work very good.
To change the flapper, turn the water supply valve off and flush the tank to drop the water level. Remove the old flapper and install the new one in the opposite manner as removing the old one.
Adjust the chain length and attach the chain to the flush valve handle.
Turn the water valve on, wait till the tank fills and test flush the toilet. Make any final adjustments that may be necessary.
