How to replace a flapper?
With a flapper, shut off the water; unhook the chain; and lift the flapper from its pegs at the base of the flush valve or from around the overflow tube if the flapper has a collar. Before installing the new flapper, clear the valve seat of any old rubber (slime) or mineral deposits. Wipe the seat rim with a paper towel; then sand it lightly with fine sandpaper or steel wool. An abrasive pad from the kitchen will also work. You may need to use a little vinegar to dissolve mineral deposits.
The new flapper will likely have two types of flush-valve attachments for a universal fit: side tabs that hook over the valve's side pegs and a rubber collar for use on valve without side pages.
Step 1: Clean the flush-valve seat by rubbing it with an abrasive material such as steel wool or a scouring pad until it is smooth. Check for imperfections by wiping your finger over the seat.
Step 2: Universal flush-valve flapper are made to fit both of the standard flush valves. If the valve has no side pegs at the bottom of the overflow tube, slide the round collar at the back of the flapper over the tube, and push it down onto the valve seat. If the flush valve has side pegs, cut off the collar using acissors and discard it (inset). Then hook the eyelets on the flapper over the pegs on the overflow tube.