Yes, the OP can change the accepted answer. Only the OP has the ability to do it. We've received flags on old questions in the past indicating that the accepted answer is now completely wrong and better answers exist, however there's nothing we can do about it.
If you check the OP's profile page and see that they've been around (relatively) recently, it's worth leaving a comment. You should also comment under the now dated accepted answer. Either the OP will accept a better answer, or the author of the answer will update it.
If that doesn't work, edit the accepted answer to make it correct. If that requires what you feel is too much of a drastic change, add a note on the accepted answer pointing to the better one.
Still, give both authors time to receive the notification unless the incorrect answer might be causing spontaneous kitten combustion.