The answer which is "accepted" by the OP is merely that - the answer which the OP most approves of at the time. It is not intended to be the "best answer" and there is no guarantee that it is even "correct".
It is true that a random searcher might wrongly assume the "accepted" answer is "correct." But that is a fault in the presentation of the feature rather than its operation - how it appears rather than what it is intended to do. (Perhaps the green tick needs to be replaced by something less evocative of official approval - a smiley face?) And no sane visitor would think an answer "correct" if it has overwhelming down-votes.
The Help Centre explains :
What does it mean when an answer is "accepted"?
Accepting an answer ... simply means that the author received an answer that worked for him or her personally. Not every user comes back to accept an answer, and of those who do, they might not change the accepted answer even if a newer, better answer comes along later.
The rules of the site allow the OP complete freedom in choosing which answer to "accept" - even an answer which is "obviously" incorrect. Probably the choice in this case was intended to be a deliberate "poke in the eye" to those who criticised the question. However, any interference with this choice by a Moderator could only be done if there was an abuse of the system, and even then could only remove the green tick and not award it to another answer. I agree with John Duffield that such a practice would stray into censorship.
Changing an "accepted answer" to ensure that it is the "best answer" would be altering the central purpose of the feature, and it would be fraught with difficulties. If the "best answer" is the one with the most upvotes the feature would be redundant. If it is the choice of some other user or group of users, then who exactly will have this privilege?
A large number of upvotes is no guarantee that the answer is "best" or even correct. As with questions, it is only an indication of popularity. Votes for answers only indicate whether or not they are "useful". This is explained in the Help Centre :
Why is voting important?
Voting is central to our model of providing quality questions and answers; it is how ...good content rises to the top...incorrect content falls to the bottom...
Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite: that the post contains wrong information, is poorly researched, or fails to communicate information. The more that people vote on a post, the more certain future visitors can be of the quality of information contained within that post...
I agree with the two Johns that poor or incorrect content should be highlighted by :
- posting a correct answer which others can up-vote.
Related Meta questions :
Why isn't bad content downvoted and commented on enough
Is it appropriate to use the low quality review system to get rid of 'bad' answers?