Turn the water supply cutoff valve clockwise to shut off the water supply to toilet. The cutoff valve is on the water line behind the toilet, usually on the left rear side of the toilet.
Lift the toilet tank lid off of the toilet tank and set the lid on a flat surface.
Remove the top of the ballcock by unscrewing the locknut with your fingers. The ballcock locknut is the piece that the float ball arm connects. Set the ballcock on the top corner of the toilet tank.There is a white plastic button on top of the rubber seal that you see after removing the top of the ballcock. Lift the button out of the ballcock and set the button aside.
Lift the white plastic button that is on the center of the seal on the ballcock. The button will pull straight up with your fingers.
Lift the two rubber seals out of the ballcock body. If the seals show any damage, take the seals to a plumbing supply store to purchase replacements.
Clean the seat area where the seals sit in the ballcock with a clean cloth. Place the seals back into the ballcock body. Set the white button back over the top of the seals in the middle.
Place the ballcock locknut back over the ballcock and turn the nut clockwise to lock the nut in place.
Turn the water supply valve on to the toilet tank. Place the toilet tank lid back on top of the toilet tank.