How do you change out old gas with out dropping tank

How come none of these questions get answers¿???????????????

You would have to siphon the gas out which is not easy due to the anti siphon mechanism.

An old school and slow way to do it is to open the fuel cap and vacuum pump it from the fuel tank side of the fuel filter. This will not get the old fuel 100% out but close to it depending on what you are planning to do next.