Questions and their answers become stagnant over time. A question could have had one answer two years ago and now another answer. Questions become stale as does the information. Do you just keep posting to the original question with the update?
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If there is a question that was asked a long time ago and answered, but there are new answers now that you think are "better" (possibly utilizing language features added in the meantime, or simply new techniques that have since become popularized) then you can simply answer the old question with a more up-to-date answer. Such behavior is explicitly encouraged by the site as it helps to ensure visitors from google have access to the best answer for a question, not just the best answer at the time it was asked.
If a question is simply no longer valid, possibly because it related to a bug/problem that was fixed with a newer version of the language, or that the requested tool has since been depricated, then the appropriate solution would be to close the question as "Too localized". Even if the question was just fine at the time it was asked, if the entire question is no longer applicable then it should be closed. (The information will still be out there and not deleted for those still using those old systems.) If you don't have enough rep to vote to close, or you don't think there will be enough traffic to get 5 close votes, then you can flag it for moderator closure.