There is no rule. Updating the thread with current information, either on top or at bottom, is most appreciated. I've been on a lot of sites where people ask questions and then never follow up with what they decided.
IMHO and from reading 1000s of posts and topics it's best to edit both the original post and also add to the topic with the new info in a new post at the end of the topic/your last reply, that way you have covered all the bases and most everyone if not all will see the edit.
The reasoning for doing it this way is some only read the original post and then reply and miss all the other post after the original post, and then some only read the last few replies and never see the original post.