Depending on how much gas is in the tank, and how long its been sitting makes a huge difference on how the replacement is handled. To pull the tank, it needs to be very close to empty. I usually connect a free standing electric fuel pump to the gas line at the front frame rail and pump the old gas into 5 gallon containers. If the old gas has been there many years, I'll empty the tank and pull the tank. It likely needs to be cleaned out, or there will be future problems with the residue contaminating the new gas for several full tanks of fuel.

If the old gas still smells like gas, siphon a little out and see if it still burns (pour a 6" diameter spot on the cement driveway away from everything and throw a match on it). If it doesn't burn, it all needs to come out (If the gas has to come out, I'm pulling the tank too). If it burns, you can siphon out about 1/2 of what is there, add the same amount of new fuel, and drive it. The burnable gas you siphoned out can probably be used in a mower or something with a small easily removable fuel tank, if used up pretty quickly. Non-burnable gas needs to be disposed of, you need to check with your local authorities to see how and where the disposal should be done. Gene