To repair a toilet valve, turn the water off at the toilet tank, and remove the lid to the toilet tank. Hold the flapper open, flush the toilet to drain the water, and unhook the chain connected to the flapper valve. Pull the flapper off the pivot ends connected to the overflow tube, set the new flapper valve into place, and rehook the chain. Replace the tank lid, and turn the water supply valve to the on position.
Another option is to replace both the overflow tube and flapper assembly. To replace the assembly, turn the water supply valve off, and flush the toilet to drain it. Disconnect the water supply line from the toilet, and remove the bolts that secure the toilet tank to the toilet base.
Lift the toilet tank off the toilet, and remove the plastic gasket and hex nut from the underside of the toilet. Remove the overflow tube, and set the new gasket and overflow tube in place. Cut the overflow tube so it is 1 inch below the fill mark on the tank, and reassemble the toilet using a new rubber gasket and new bolts fitted with rubber washers. Turn the water supply on to check for proper operation.