It's a difficult process that changes greatly from model to model, and often comes with... Unexpected... Functionality loss.
Like for a particular VW model I'm thinking of, you need to disassemble the entire front of the car to get at it, and it disables the seat warmer for no obvious reason.
Unless the car is phoning home to enable options you've paid for, you might be able to evade anti-functionality issues by replacing the antenna with a dummy load (resistor) rather than by removing the modem entirely.
