This is a recurring discussion on StackOverflow and the whole StackExchange network and I don't think there is a definite official answer.
Some threads on Meta.SE
- if obsolete answers can be easily fixed without changing them completely (like replace a dead link), edit the answer
- otherwise downvote and add a comment to notify the author and other readers (If I am 100% sure that the answer is not useful anymore you could even edit a notice into the answer itself).
- add a new up-to-date answer if necessary
In case of the linked question How to download magento
I already downvoted and commented three of the answers in the past for that very reason. The top voted answer by @FabianBlechschmidt at least works partially: the
curl method, not the direct link. So comment and/or edit would be appropiate. If you have a completely different solution, feel free to add a new answer.
And yes, a new question would be a duplicate.