How do I change Toyota Pressure Switch?

I need to change my A/C pressure switch for my 03 Toyota Camry. I tried to take it off but the freon started to come out. Is there any way I can prevent this so I can replace it. I really dont wanna have to take it to get done to get over charged.How do I change Toyota Pressure Switch?you will have to take it and have it charged no matter what...too bad