turn the water off to the toilet then flush it.disconnect the water supply line then on each side of the tank on the bottom you will see a big screw and a nut.take them off.then pull the tank off.in the middle you will see a big rubber gasket take that off then the big nut.unscrew that and that will remove the overflow tube.put the new rubber gasket on the bottom of the new overflow tube the put it back in and tighten the nut.make sure you put it back in in the same position as the old won was in .you will need a new tank to bowl gasket that goes over the nut and a new set of tank bolts.when putting in the tank bolts start with the screw,metal washer,rubber washer.put it in the hole.the same with the bottom only in reverse.slide rubber washer over screw and then metal washer then the nut.