Siphon and a fuel additive. The older tanks have a clean out hatch, though the newer ones are not accessible through a hatch. Some folks use the sending unit for access. If it is the 33 or 27 gallon belly tank with the offset in the bottom, then you can probably just drain the sump through the pick up and siphon or pump it out. Electric automotive fuel pumps work for this, though be aware of fire dangers and concerns of sparking. BCB