I think the best option is to initially leave a comment saying "Hey, as of XX/YY/ZZ this is out of date. Would you mind updating?" That way, anyone who is looking quickly can see this isn't accurate anymore, and you give the answerer a chance to edit in new information. After a day or so, I'd say it's fine to downvote (and if you'd like, leaving a comment about how it's no longer accurate). If you know the current answer, you could make a new answer.
The initial comment is by no means required, but I think it's a nice gesture. An example of this in action would be one of my Destiny answers - a significant system in the game was recently overhauled, and one of my previous answers was completely incorrect. Thanks to a comment, I was notified and updated my answer to reflect the new state of the game. Honestly, I barely even remembered answering that question (it was asked several months ago), so without the comment my highly voted but no longer accurate answer would have probably lingered for a while.