Jack, hmm. I just checked the iOS version and they added an Edit feature in a story since I last checked. But only when you access the story from the Stories page via your Profile page. Not if you just select a story normally. Weird.

On iOS (iPhone and iPad,) if you select a single word, the pop-up gives you these options: Highlight, Respond, Share, and three tiny circles. If you click the three circles you get: Define, Copy, Cancel. No Edit.

I will leave my mobile editing comments, for now. With a note that they apply to iOS only.