You can purchase a replacement flush valve in every do-it-yourself store or you can call a plumber, who can replace the old flush valve and install the new one.

Easiest way would be to replace the flush valve, otherwise there is a washer underneath that is easily replaced

How do you back flush a pool filter?

More commonly called the 'flapper', it 's the valve that fits over the hole in the base of the tank that you activate when you press the flush lever.

Either the flapper is letting water through or you need to replace your flush valve or there is something stuck between the stop and the flush valve.