I think (which is dangerous) that you have to go into the ORIGINAL post, and find EDIT button. Click!
Then, Believe you can change those faces/emotioncons that way. You want the ones OVER the edit screen, NOT to the left. The ones on the left go IN your posts, like this
I am really shocked.
I changed the one on my reply to a thumbs up, to show what happens when I click on something OVER the edit screen.
Probably clear as mud. LOL