WHY DO THIS?
A running toilet can waste two gallons of water per minute. That will do a number on your water bill this month. Plus, if you don’t repair it, your toilet can spring a major leak which can cost thousands of dollars to fix. And, of course, it makes a really annoying noise!
First, take off the toilet lid.
Flush the toilet. Pay attention to your chain and flapper. If your chain is tangled, that’s your problem! A tangled chain prevents the flapper from completely setting on top of the hole at the bottom of the toilet bowl. Untangle your chain, and flush your toilet again. If the flapper sits flush, your problem should be solved.
If your flapper doesn’t sit flush on the bottom of the toilet tank, you need to tighten your flapper. Do this by reconnecting the flapper to the bracket by tightening the pegs holding the flapper down. You can tighten the pegs with your hands.
If your flapper is secured, make sure it isn’t soft and spongy. If it is, it’s time to replace it. You can purchase a new flapper at your local hardware store for less than $10.
To replace your flapper, first turn off the shutoff valve. It’s located on the wall behind the toilet. Flush the toilet to remove the water in the bowl and then remove the old flapper by pulling it off the pegs on the bottom of the tank. Remove the chain from the old flapper with your needle nose pliers. To install your new flapper, simply reverse the steps to remove the flapper.
Turn your water back on by turning the shutoff valve back on. Let your toilet’s tank fill with water, and then replace your tank lid. Voila!
SOURCE: https://brightnest.com/todos/fix-a-running-toilet