How do you change the air conditioning freon in a 2000 Ford Fiesta?

It does not contain Freon. It contains R134a. There is no need to change the refrigerant. Any A/C service should be done by a qualified technician, with the proper tools and equipment. Please do not attempt any service or repair without the equipment and expertise necessary. Serious burns due to frostbite can occur through contact with the refrigerant.