Newest top left, as on your profile. Your current posts follow, faded, so you see the new ones in context. Tap a post for its photos, caption and music.
To reorder: drag a post onto another (they swap dates), or change the date inside the post.
To plan something new (post, carousel, reel or story): tap “+ Add”. Open a post to change its photos, video, title or caption directly. Changes save on their own; “Undo” goes back a step.