This isn't really an option at the moment -- at least not for me, and judging by posts I've seen on this forum, not for many others. Yesterday I did indeed see a notice pop up on my phone about 2021.36.5.1 being available, despite the fact that I was on 2021.36.5.2 -- that is, it was a downgrade that, some posts here reported, would have removed the FSD beta features. As I was away from home at the time I saw this notice, I did not activate the "update." By the time I got home, the notice of its availability had disappeared from my phone. Thus, short of hacking my Tesla, I cannot change its firmware version. The most I can do is to not use the FSD beta features -- but if the problem is with non-FSD functionality, like AEB, then not using FSD beta features won't help.
If there is a safety flaw in the latest firmware, then it's Tesla's responsibility, not mine, to push out an update ASAP. It's owners' responsibility to accept and install that update, of course, but Tesla's software-distribution method means that we have no ability to downgrade on our own. At this point, the best I can do is to minimize my driving. This is the downside of Tesla's OTA software updates.