Is there a policy to modify the answer of an OP, when he obviously picked the wrong answer. Or will the correct answer get to the top when it gets enough votes?
No, there is not.
Only the OP may change the accepted answer because the "accepted answer" is defined as the "answer which worked for the OP".
If the accepted answer is wrong or incorrect, voting will determine it's actual credibility. There are many cases in which an unaccepted answer has higher votes than the accepted one, which is perfectly fine.