Before removing the old flush valve, make sure you shut off your water supply and flush the toilet to empty the tank. Use a sponge or rag to remove any remaining water at the bottom of the tank. Place a bucket under the fill valve to catch any remaining water.
Remove the supply line from the fill valve and remove the flapper chain from the flush lever. Disconnect the refill tube from the overflow pipe and unscrew the bolts connecting the tank to the toilet bowl, again using a bucket to catch any remaining water drops.
Under the tank, unscrew the mounting nut holding the flush valve in place. Take out the old flush valve and replace it with the new one. Tighten the new flush valve in place and reattach the toilet tank.
Reattach the flapper chain to the flush lever and the fill tube to the new overflow pipe. Reattach your water supply to the fill tube, turn on your water, and test for leaks.
If you are unfamiliar with plumbing, we recommend consulting a professional.