If your toilet is constantly running, this may mean it is time to replace the fill valve.
Before removing the old fill valve, make sure you shut off your water supply and flush the toilet to empty the tank. Use a sponge or rag to remove any remaining water at the bottom of the tank.
Remove the supply line from the fill valve shank and place a container under the fill valve mounting nut to catch any dripping water.
Remove the nut and take out the old fill valve. Replace with the new fill valve, making sure that the new valve is at least 1" above the toilet overflow tube.
Reconnect the fill tube to the overflow tube and the water supply hose to the fill valve shank. Turn on your water and test for leaks.
If you are unfamiliar with plumbing, we recommend consulting a professional.