How to repair a toilet that won't stop running?
Remove the tank lid. Examine the flapper, which is the rubber mechanism, usually red or black, that seals the water in the tank when seated, then allows water to drain from the tank when raised. Determine if a problem with the chain that runs from the flush lever to the flapper is preventing the flapper from seating properly. Add slack to the chain if it is too short, not allowing the flapper to seat fully, or remove excess chain, using a pair of snips.
Turn the shutoff valve behind the toilet completely clockwise to shut off the water to the toilet, then flush the toilet. Remove the damaged flapper from the tank, unhooking the flapper ears from the pegs on the bottom of the overflow tube and disconnecting the chain from the flush lever.
Cut the replacement flapper along the line marked "CUT" with scissors to remove the collar. Slip the flapper ears over the pegs on the bottom of the overflow tube. Hook the chain onto the flush lever, leaving the proper slack for the flapper to seat properly in the closed position. Turn the shutoff valve counterclockwise to refill the tank. Flush the toilet and observe whether the flapper rises and seats properly.
Determine if water is rising and spilling over into the overflow tube. This signals a faulty fill valve. Slightly bend the float arm down, to lower the water level. If your tank has a water-intake assembly instead of a float ball, pinch the clip attached to the thin metal rod and slide the clip and float cup down on the fill valve. Flush the toilet and determine if the water refills to a proper level.
Turn the shut-off valve completely clockwise, shutting of the toilet's water supply, if you observe that the fill valve is damaged and needs replacing. Flush the toilet and hold down the handle in order to remove the most amount of water from the tank. Absorb the remaining water with a towel, then place the towel beneath the tank. Unscrew and remove the supply tube from the bottom of the tank, using an adjustable wrench. Remove the fill valve. Position the replacement fill valve in the tank. Hand tighten the supply tube to the bottom of the new fill valve, beneath the tank. Tighten the supply tube an additional 1/2 turn, using the wrench. Turn the shut-off valve counterclockwise and check for any leaks.