There are 2 ways.
1. If your are converting it to R-134 then just tear up your dashboard and take it oud and use a 3M coil cleaner.
2. If you do not want to mess up with the freon. You would have to carefully open up the area where it is installed. If you can utilize the drainage of the HVAC box, then carefully using water and the coil cleaner you can still clean it. Also, there are lots of coil cleaners for cars availabe in the market which claim to clean the coil by just drilling a hole in the HVAC. Never tried it.