To be clear, in a Discourse forum, this is restricted to moderators/admins. It is not something the user can do.
###Requesting Changes
Flagging doesn't seem to be a very good way to deal with anything other than problem posts, since, per the Discourse Co-Founder, the post is immediately hidden when flagged. If you want some structural changes to your topic/post, then I suppose the best course is to PM any of these: