It's not so easy. The Engine mount needs to be removed and the engine jacked up a little bit to get the motor out.
If you do this job, then make sure it's done properly with a quality replacement. It's not something you want to be doing on a regular basis.
Expect the dealership to charge over £1000 to do it and also the OE motor is not cheap (about £700)
I paid £700 for an Audi approved garage to fit a quality pattern part 3 years ago, I'm going for the OE part this time at the dealership with a 4 hour labour cost on top.
From Elsa Win how to change an RS4 starter motor. Had a quick look on Autodoc you can get a remanufactured Valeo starter motor for around £150. A new Valeo starter just over £500.
I believe that Valeo is probably the manufacturer of the OE part.