Please tell me how to replace the gasket on the flapper assy.

Flapper or flush valve needs servicing
First try cleaning minderal deposits from around the valve seat. This can keep the flapper valve from sealing properly and allow water to seap into the bowl. First turn of the water and flush the toilet to empty the bowl. You can unhook the flapper valve to get it out of the way. Using emery cloth scrub the inside of the valve seat to remove the mineral deposits. Next coat the valve seat with petroleum jelly to help the flapper valve make a tight seal. Replace the flapper valve, turn on the water and check to see if the water still leaks.
If the flapper valve is old it may be deformed. In this case you will need to replace it. You can unhook the valve and remove the lift chain from the valve. Be sure to purchase a similar size replacement valve.
If the valve seat is damaged you will need to replace the entire flush-valve assembly. You will need to remove the tank to do this. Be certain to turn off the water supply and flush the tank to drain it. You might also want to place newspaper and towels on the floor to soak up any water that may leak out. Using and adjustable wrench loosen the nuts at the supply tube and push the tube aside. Next, use a flat-head screwdriver to hold the bolts inside the tank still while using an adjustable wrench to loosen the nut on the bottom of the tank. You will then be able to lift the tank off of the bowl. Lay the tank on the newspapers or towels.
To replace the flush-valve assembly first remove the old one by unscrewing the locknut on the bottom of the tank. Use a hacksaw to cut the new overflow pipe to 1/2 ince below the tank top. Put plumber's putty on the washer at the base of the flush-valve assembly and push the assembly against the tank opening, removing any excess putty. Use a monkey wrench to tighten the locknut. Replace the gasket over the locknut and replace the tank on the bowl and reassemble them.
g and adjustable wrench loosen the nuts at the supply tube and push the tube aside. Next, use a flat-head screwdriver to hold the bolts inside the tank still while using an adjustable wrench to loosen the nut on the bottom of the tank. You will then be able to lift the tank off of the bowl. Lay the tank on the newspapers or towels.
To replace the flush-valve assembly first remove the old one by unscrewing the locknut on the bottom of the tank. Use a hacksaw to cut the new overflow pipe to 1/2 ince below the tank top. Put plumber's putty on the washer at the base of the flush-valve assembly and push the assembly against the tank opening, removing any excess putty. Use a monkey wrench to tighten the locknut. Replace the gasket over the locknut and replace the tank on the bowl and reassemble them.