In this suggested edit I noted that the SIGAR library is now licensed under Apache v2 and not GPL as is currently mentioned by the answer. Normally I would leave a comment in the answer hoping that the user who posted the answer might edit in the update, but in this case that user hasn't logged in to SE for 4 years. Did the way I posted the suggestion actually change anything in the meaning of the original post? If so what is the best way to post such an update?
Correcting a factual mistake that is not a crucial element of the post is definitely a good thing. It's even more clear-cut when the answer was correct but some minor part of it needs to be updated: yes, please do update. In a case like this one, you don't need to leave a comment, go ahead and edit the information in.
Your edit was a good idea in spirit but not in execution, because it breaks the flow of the answer. That the license has changed at some point is extremely minor information, but with your suggestion it's the most prominent part of the post. Just change the sentence that needs changing, and be sure to explain that the situation changed in the edit description.
Never add something like “Update: XXX” when you edit somebody's post: either it can be integrated in the flow of the text (that's the case here), or it should be a comment (that's what the reviewers who rejected your edit decided, with just reason).