Your best bet is the comment on the question and bring it to the attention of the person that asked the question. In the end it is up to the person asking the question to pick the answer that help them the most or that is the most correct.
As for question if it is completely out of date and no longer relevant at all then flag it to be closed with a comment explaining why it is no longer relevant. A good example of this would be a question about SEO that was once valid but Google has no completely invalidated by changing their search algorithms.
You should answer it correctly with up to date information. The ebb and flow of visitors will even out the correct answer over time. I've taken the initiative for you. Four votes, and order will be restored upon the universe!