Any user has the ability to suggest an edit. This has exactly the same interface as editing the question, but it means your edit is reviewed before it takes effect. People with more rep on the site can check your edit, and the original answerer gets a notification that an edit was suggested on his post: he has a veto and can immediately accept or reject the edit, taking it out of the review system completely.
If you don't know what the edit should be, you can comment on the answer as egid suggested, and hope for the answerer or another user to add more information.
Alternatively, you can bounty the existing question, giving up some of your rep to encourage users to revisit it. The bounty can be awarded on new or existing answers, meaning that someone who previously answered it can edit their answer and be eligible. One of the standard bounty reasons (you can enter one when adding the bounty) is that the existing answers are out-of-date and need updating.
One last option, if you don't know and you don't have the rep for a bounty, or if you're just not sure whether the answers need to be updated, is simply to come into [chat] and complain that the answer is out of date. Enough knowledgeable and high-rep users hang around chat that either one of them will update the outdated answer or one will bounty the question as described above.