This has already been discussed [extensively](http://meta.stackoverflow.com/questions/145301/what-can-be-done-with-a-blatantly-wrong-answer-to-a-popular-question) on [Meta.StackOverflow](http://meta.stackoverflow.com/questions/78438/blatantly-wrong-accepted-answer).

Downvoting may not achieve much, because of a very simple reason: users who are most likely to arrive at that question's page are those less familiar with the topic (hence them seeking an answer). They're therefore quite likely to just upvote the top answer and run with it. Here's [an example](http://stackoverflow.com/questions/6579844/how-does-zalgo-text-work), an answer that has been wrong for 2+ years until I bugged a moderator enough, who in turn bugged the OP, who finally edited their answer.

I would contact the OP and ask them to edit the answer. If they don't, and there is solid agreement that the answer is wrong - be bold and edit it yourself. I cannot find any rule (such as not "drastically" changing the meaning) to ethically justify spreading factually incorrect knowledge.