You will have to pull off the fabric upholstery from the baggage bulkhead. It is held on by strips of Velcro. There may also be a couple of screws at the bottom edge. You have to bend the stiff upholstery panel and work at it carefully to get it off. Then there will be a number of phillips head screws holding on the panel. Be careful removing those and don't drop any behind the fuel tank. There are stainless and non-magnetic and you can't reach in there with a mechanic's magnet to retrieve them. I spent more time once trying to get back a dropped screw than taking everything apart to get access to the tail-cone!
The ADAHARS is up on the top of the tail-cone and pretty easy to get to. You do have to squeeze through the opening if the fuel tank is in place, and you will have to lie on your back to work. There are control cables running back to the tail. You could probably arrange 2 or 3 moving blankets to pad yourself and keep pressure off the cables. I made a little floor panel out of scrap because I have been back there a few times now and wanted a little more comfort. It rests on the first bulkhead back and it has slots to keep my weight off the cables.
The ADAHRS will have one electric D-sub and 3 tubing connections. Mark the tubing so as not to mix them up. The ADAHRS is held on to its mounting bracket by four small non-magnetic screws, nuts, washers. Be careful not to drop these small parts as they will be hard to find.
Try to figure out what tools you will need before going in. It's a real pain to get back there and need something. If I remember correctly you need a short #2 phillips and a small metric combination wrench, forgot the size, take a couple of your smallest. Have a helper on the outside.
I have put a padded sawhorse under the tail tie-down ring so if the plane wants to tip back from my being in tail-cone it won't go all the way to the hangar floor. I weigh about 190, it has never dropped back.
I hope you don't have to remove your ADAHRS but this may help if you do.