Depends what gas your running??
Is it a R134a system? If so i think???? you might be able to do it yourself.
If it is R12, you have to get it degassed at a qualified place, since it is a CFC and its bad for the atmosphere.
Have a look at this forum. It has got all the info you'll ever need on doing anything related to automotive air conditioning. (Tells you how to regass it too, if your game!)