You probably have to change out the whole console. For my a4, I had to remove the center armrest to get to the center console. then remove the center console to expose the cage that holds the radio and ac unit. Then swap out the single din cage with double din cage. Shave down sides of AC unit to be able to fit it lower.
I would imagine the a6 requires similar or more work.
It is definitely a lot of work, but well worth it to get nav, bluetooth, movies, etc.