Get a specialized giant tire pneumatic bead breaking tool. Be sure you have the proper pneumatic equipment such as high pressure hoses and a tank so you can use your bead breaker.
Attach the bead braking tool to the rim of the tractor tire according to the manufacturer's directions.
Break the bead. The "bead" is how the tire is set on the rim. It takes a great deal of force on the sidewall of a tractor tire to break the bead.
Pry the tractor tire off the rim using a large tractor tire derimmer. A suitable derimmer can be purchased from Eagle International (see Resources below).
Actuate the cylinder on the tire derimmer to eject the rim from the tractor tire. Be sure to clean and inspect the rim after you remove it for damage before trying to set a new tractor tire on it.