This was addressed in this [**Mother Meta post**](http://meta.stackexchange.com/questions/8879/). The answers there basically say there are three things you can/should do:

 1. Downvote & comment
 2. Provide a new, correct answer
 3. Flag for moderator attention (reserved for *dangerously* incorrect & *doesn't answer the question* answers; though neither really make sense in context of a highly upvoted answer)

Editing isn't discussed at all in that Meta post, which suggests that it shouldn't be done. I also have [argued in the past](http://meta.physics.stackexchange.com/questions/6010/) about **not** correcting incorrect answers (except for the case of a typo, as DavidZ mentions in a comment on my post there).

However, dmckee did mark the post in question as Community Wiki, giving control over the post to the *Physics.SE community* to own & edit. I cannot find anything in Mother Meta about this particular case (links welcome!), which suggests that it's probably pretty rare (read: handled case-by-case per site) or may not have happened before (read: no network-wide precedence).