Notes on Nostr are not editable. Same reason transactions on the Bitcoin blockchain are not once they’re confirmed.
Discussion
I can edit notes with other clients.
What client's offer that feature? I've only used Primal and Damus.
More than likely what they're doing is a "local edit" where they show the changes in that client but were you to use another client/relay combo the changes would not be present.
I’ve seen that feature on Amethyst and few other clients