Where does it do this?

There are thumbs up and thumbs down to the right of the response, next to the clipboard.

If you hover over your message, an edit button appears. Editing your message fork the conversation, and creates a pagination thing to the left of the user icon.

Oh, sorry, this falls short of what I was asking for:

For the thumbs, this does not seem to change the conversation in any way - ChatGPT does not react.

For editing (this is new to me, so TIL, ty!) - it seems to share memory across threads in a few naive tests I just did, so almost there, but not entirely.

You can edit a message you’ve already sent by hovering over it and clicking the “edit” icon, which will create a new “fork” in the conversation. You can toggle between forks as needed too.

I dont think you can jump back though, right? If you change a previous prompt, you’re just resetting to that point and creating a new thread at the expense of the old one? I use this feature but have coke to the conclusion this is how it works. Have you seen differently?

Once you've changed a prompt, a pair of arrows show up to the left of the prompt allowing you to toggle between the new and old prompts!