Turn off the water supply valve behind the toilet
Remove the tank lid and set it aside carefully
Check the current water level against the fill line or overflow tube mark
Locate the fill valve adjustment screw, clip, or float arm
Turn the adjustment screw clockwise to lower the water level
Turn the adjustment screw counterclockwise to raise the water level
If the tank has a float cup, slide the clip up or down as needed
If the tank has a ball float, gently bend the float arm to adjust the level
If the tank has a floatless fill valve, turn the top adjustment screw to set the level
Restore the water supply and let the tank refill
Flush the toilet and recheck the water level
Repeat the adjustment until the water sits about 1 inch below the top of the overflow tube
Replace the tank lid once the level is correct