There are many Q&A on the site which were about some feature the OS didn't have at the time and therefore the accepted answer is NO, something something, ...
Now that changed with the new OS release and that feature IS available, if you have OS version X.Y. But those Q&As are not being updated.
What is correct way to handle those Q&As?
Comment on the Q or A seems very inappropriate to me. It can be overlooked and generally shouldn't be a source of information. Also may contain lots of information in those cases.
New answer on the question seems inconsistent as there would be always two correct answers, mostly with the older accepted (many of old OPs will not cooperate on this as they may not visit the site again), which would not be ideal in the long run.
Maybe create new question, tagged and formulated in the new OS version-way and cross-linked with the old question (and retag/reformulate the old question in the old OS version-way).
What do you think? Any other ideas?
And some examples: