How do you recharge the air conditioner

This is probably R12 refrigerant - beware that this refrigerant - if you can find it - is very costly. You will want to consider a retrofit kit to the newer R134a refrigerant. As always, refrigerant handling requires EPA section 609 certification by qualified technicians.

If in fact the system is low and not cooling then you have a leak and should have dye added to locate the leak with a blacklight at a qualified shop.