There's 2 drain bolts of the rear diff. I was told to fill one of them I need a machine. Has anyone has any experience doing fluid changes yourself

 

I haven't done it on a Audi B8 before but I've done trucks, to subarus and their all almost identical.
I can't imagine Audi being any different.
The "Machine" your referring too I think (yoda correct me if im wrong) is just a pump to push the oil in.
You can do the same with a hand help 19.99$ gun at CT or a bottle and clear tube.

Your just draining and pushing new fluid in at the end of the day.