I have created a question on how to have "synonyms" context menu in Libreoffice Writer. I was concerned mostly by British English so, a comment to this question was the correct answer. But in fact that answer already existed to a similar question specifically asking about English UK (here). Only that it was not the answer that was selected as definitive in that case, while the one selected doesn't in fact work (anymore). How to confirm and highlight what the correct answer is?
The best thing you can do is leave a comment on the incorrect answer indicating that it no longer works and vote up the correct answer. You could also leave a comment on the question for the OP - and he can verify that the new answer works and mark it as accepted if he is still around.
Since the existing answer is just a link to another post that then links to some blog that is out of date, it's clearly incorrect.
As George indicates, a courtesy comment is nice, but at the end of the day if it's wrong, vote accordingly and the author will either fix it, or delete it.
Personally, I think the accepted answer is not that good even when it was correct. As for your original question we should mark that as a duplicate of the original and then just concentrate on making sure the correct content floats to the top.