What is the replacement for freon in air conditioning system?

"Freon" is a trademark of DuPont for their CFC and HFC refrigerants. Thus, if you actually have Freon as manufactured by DuPont, then the replacement would be the same type of refrigerant manufactured by any other manufacturer. For example, if you had a car with an R-12 (CFC-12) air conditioning system, then you would only replace it with R-12. Same thing if you have a car with an R-134a (HFC-134) system - you can only replace it with R-134a. Never mix refrigerants and never use a refrigerant other than the type specified for your system.