Turn off the water supply valve behind the toilet
Remove the tank lid and set it aside safely
Flush the toilet to lower the water level
Locate the fill valve adjustment mechanism
Turn the adjustment screw or slide the adjustment clip to change the water level
Raise the setting if the water level is too low
Lower the setting if the water level is too high
Check that the water stops about 1 inch below the top of the overflow tube
Turn the water supply back on
Let the tank refill and observe the water level
Repeat the adjustment if needed
Replace the tank lid once the level is correct