How do you get rid of old gas in gas tank?

I Think you have other problems. Change the fuel filter and if that doesn't do it take it to a garage.

You purchase a gasoline can and a funnel. Fill the gasoline can with fuel from your local gasoline station. Pour it into your vehicles tank using the funnel. Cycle the ignition key on and off several times pausing breifly for a few seconds when in the on position, this will prime the system, then start it up.