Get a siphon (kerosean siphon works great) and suck as much out you can. If the current fuel in the tank is bad, make sure you purge the fuel lines and drain and clean the carb bowls before attempting to start after refueling or you could end up with cloged circuts in the carbs and burn her down.
Also add some good fuel system conditioner in the first new tank. Use something like Seafoam fuel conditioner.
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