The fill valve inside your toilet tank determines the amount of water it will release when flushing. If your toilet is not flushing completely, or the toilet is running all the time, adjusting the fill valve will be necessary. Older fill valves have the ball float that attaches to the valve with a metal rod. The newer fill valves use a float that rides up and down the shaft of the fill valve. Adjusting the fill valve is a matter of repositioning the float level.
Remove the toilet tank lid from the toilet. Set the lid on a flat surface out of the way.
Locate the screw that secures the float arm to the top of the fill valve, if you have a ball float. Turn the screw clockwise with a flathead screwdriver to raise the float if the toilet is constantly running. Turn the screw counterclockwise to lower the float to add more water to the tank, if there is not enough to flush the toilet completely.
Grab the float arm near the fill valve with a pair of pliers, if the adjustment screw does not resolve the float level. Turn the arm counterclockwise to remove it from the fill valve. Bend the middle of the arm up or down as necessary, and reinstall the arm into the fill valve. Do not bend the arm to make adjustment to the float while it is connected to the fill valve. Bending the arm while it is installed could cause damage to the fill valve.
Locate the pinch clip near the side of the float, if you have a fill valve with the float on the shaft. There is a small metal line that the clip surrounds. Pinch the clip with your fingers, and move the float up or down as necessary. Release the pinch clip. Place the toilet tank lid back on the toilet.