Generally speaking six months is considered the safe window for fuel in your tank, so it is possible that the remainder of the fuel now in your tank is beyond the point where it is safe to use. Filling up the tank with fresh fuel should be sufficient to dilute the older fuel and make it safe to use, but if you don't drive far enough to use the petrol then eventually you will be back in the same position. One option would be to use a fuel stabiliser additive which would help to keep the fuel usable for longer, alternatively you could make some regular journeys on petrol only to ensure that the engine is running and the fuel in the tank is used and replaced on a regular basis.