Drain stale gas

There's no drain plug on the 2nd gens due to the bladder that's used on the fuel tank. The easiest way to drain gas is to unhook a fuel line under the hood and then one way or another, feed voltage to the fuel pump to get it out (need to purchase wiring diagram). I don't believe it's possible to put a line into the tank via where you'd normally fill as I think there's a screen. Only other option is to drop the tank out of the car.

That being said, the fuel bladder on the Prius is incredibly good at not letting air in or out, which should significantly extend the life of gasoline in the tank. How old is the fuel in the car? The normal shelf life of gas is 6 months, but will probably be okay for 2x that in the Prius due the bladder. You might very well be okay with just filling the car up with Premium and using the gas that's in it (octane rating of gas drops over time, so Premium will help equalize it).

 

More info needed.