What's the etiquette for correcting answers?

Question

I've noticed some accepted answers with a lot of votes which are strictly speaking not correct, even though they contain a lot of useful information.

Just as an example, the current accepted answer to this question is mostly pretty good however the first sentence is very wrong - a certificate isn't any kind of public / private key pair.

Given that one of the ideas of stackexchange sites is to surface 'canonical' answers to such questions which anyone can find using google, what's the best way to deal with this kind of thing?

Should we

1. comment that the answer is wrong

2. edit the answer ourselves if we can (even if it's already been accepted and even if it needs a lot of edits?)

3. downvote the accepted answer and add a new, better answer (even if the existing answer is 90% OK but wrong only in some detail?)

4. some combination of the above

5. something else?

I realise this is really a question that could apply to any stackexchange site but I guess it's possible that different sites will have their own ettiquette for this, and it's probably worth documenting what we consider to be the 'right way' here anyhow.

Please add your preferred way to handle it here, and/or upvote a solution you like.

Answer 1

The answer really is (4), depending...

• Start by commenting. This will often lead the poster to correct it himself. Sometimes even remove his answer completely, if it's totally off.

• If it's a small detail, and the poster didnt fix it (or reply), then go ahead and edit it yourself.

• If it's completely wrong, then downvote, comment, explain, and add your own answer (if you have one).
  An edit shouldn't completely change the meaning of the post.

• If the accepted answer is very wrong, and you cant get it changed that way, comment on the question itself.

And, if all that doesn't work, go back and look at the FAQ - an accepted answer doesn't necessarily mean it's the correct one. Painful as it may be.
But your comments will be noticed, at least...