When I wrote this question about The Reckoner Trilogy, there were only two books. By the time of the accepted answer, the third had been released, which contains the answer, but the answerer chose to write his answer according to the two that had been released to avoid spoiling it. How should I handle this case where the question has been superseded by sequels?
From the point of view of an answerer:
Post a new answer.
This is what I've often done in the past: when I find a question whose current answers are outdated and made obsolete by the release of newer material.
E.g. see Is it true that The Doctor can only regenerate 12 times? This is one of the oldest questions on the site, and the answers provided in 2011 were all well and good in 2011. After the release of Series 8 and 9 in 2014 and 2015, the question was in need of an updated answer, so I posted one. Luckily the OP was still active on the site, and able to change acceptance to my answer instead of an older one. This isn't always the case; sometimes you find a question with outdated or non-canonical answers, provide a perfect answer with nice canon quotes, and it languishes at the bottom of the list because nobody ever notices it and the OP never accepts it. This problem has been raised before on meta, with no entirely satisfactory solution found.
From the point of view of an asker (if you don't want to follow the above advice and self-answer):
Start a bounty.
One of the possible 'bounty notices' you can put on a question is:
Current answers are outdated
The current answer(s) are out-of-date and require revision given recent changes.