It’s not that hard if you remove the bottom trim, but if you want to do it without removing the bottom trim it will be quite the task since the opening to access the bolt does not line up with the bolt and so it is almost impossible to get the socket lined up unless you have specific tools.
I would recommend taking apart the bottom portion of the trunk area.. I wish i took some pictures but I was so into trying to finsh i forgot..
here is a thread on how I removed the bottom trim:
Since I haven't seen a basic walkthru I'll just have this here for reference for those of you who are curious what steps are needed to get this done. 2021 Model Y Performance build date of 9/21 Part# 1537264-00-B Price: $152 after tax from Tesla San Francisco Tools: flat head screwdriver...
teslamotorsclub.com
Once you remove the trim to access that panel, you need to go to the rear door area and remove the weather stripping and then you can yank the whole panel off and then be able to access the bolt easily.
This is important as you need to tighten and torque the bolt and align the seat belt anchor correctly.