Turn off the water supply for the toilet. Follow the hose from the tank to the wall where it meets the water supply valve. There should be a round or circular knob on the front of the water supply valve. Rotate this knob clockwise to turn off the water.
Remove the toilet lid. Flush the toilet to remove the water inside. Wipe up any excess with rags and sponges.
Place a towel on the ground underneath the tank. Unscrew the supply hose coupling from the underside of the tank with the pliers. Remove the plastic retaining ring above the coupling by unscrewing it.
Remove the toilet valve from the tank by pulling it straight up and out. Place a thin coat of silicone grease around the perimeter of the flush valve socket in the bottom of the tank. Insert the new one-piece flush valve into the tank and through the valve socket. Hold this with one hand.
Take the retaining nut in your other hand and thread it onto the bottom of the flush valve protruding through the bottom of the tank. Tighten this with the wrench.
Wrap the threads on the inlet valve with plumber's tape. Make at least two turns around the threads. Trim off any excess. Attach the supply hose coupling to the inlet. Tighten with the pliers.
Restore the water supply by turning the supply knob counterclockwise. Allow the toilet to fill. Flush and check that the flush valve is operating properly.