I accidentally downvoted an answer that I really meant to upvote, and didn't realize it until it was too late. (The answer not only gives the desired result, but includes a good explanation, so a downvote clearly isn't warranted.)
The system says that I can't retract the downvote until and unless the answer is edited. And I don't think that means I can change the vote if I edit it myself. And even if I could, that wouldn't reverse the change in rep that I suffered because of the downvote.
I'm not sure that flagging the answer is the right way to go, but I think that only a moderator (or someone higher in the hierarchy) can do anything about that.
So what is the proper action?