Turn off the water supply to the toilet
Flush the toilet to empty the tank
Hold the flush lever down to drain as much water as possible
Disconnect the water supply line from the bottom of the tank
Remove the old fill valve locknut from under the tank
Lift out the old fill valve
Set the new fill valve height to match the tank
Insert the new fill valve into the tank opening
Tighten the locknut under the tank
Reconnect the water supply line
Attach the refill tube to the overflow tube
Turn the water supply back on
Check the water level and adjust the fill valve if needed
Test for leaks around the connections
Flush the toilet to confirm proper operation