on the bottom of the tanks there should be a water supply hose going to the bottom of the flush valve.take the hose off then tak the nut thats holding the flush valve in off.there will be a rubber washer that goes on the bottom of the new flush valve,put that on then stick the flush valve down in the hole in the tank.put the nut on and tighten so the valve don't move,then put the hose back on.i don't know what kind you bought but you will have to adjust the water level so it don't fill over the overflow tube.there will be a adjustment screw on the flush valve on the top that you can adjust the water level.if its a fluidmaster there will be a plastic rod going up and down ,at the top you will see a place where you can adjust the water level with a screwdriver