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Edit it with a notice that it is wrong.
I think it's best to edit it with a note that it's wrong. If you don't do this, it won't be bumped to the top for the person to see.
I agree with fpqc's approach. I'm pretty sure the software actually won't allow you to delete an accepted answer.
It would also probably be nice to leave a comment on the actual post, so the person who asked the question will get a notice. This does not happen with edits.
I don't even know what post you're talking about. I just added that as an afterthought (so if people look this up in the future on meta, they'll see that as well).
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