Turn off the water supply to the toilet
Flush the toilet to empty the tank
Remove the tank lid and set it aside
Sponge out any remaining water in the tank
Disconnect the water supply line if needed
Remove the tank from the bowl if required for access
Unhook the flapper or disconnect the flush chain
Unscrew the flush valve locknut from underneath the tank
Remove the old flush valve assembly
Clean the tank opening and sealing surface
Install the new flush valve with the gasket in place
Tighten the locknut securely without overtightening
Reconnect the flapper or flush chain
Reattach the tank to the bowl if removed
Reconnect the water supply line
Turn the water supply back on
Let the tank fill and check for leaks
Adjust the chain length and water level if needed
Test flush the toilet several times
Replace the tank lid