On Nostr, the best you can do is request deletion, and relays do not have to comply with the request. Most will, though. However, your deletion request will only go out to the relays you write to. If anyone has replied, reposted, or otherwise rebroadcasted your note to other relays, they will not even receive your deletion request and your note will remain up.

This is part of how Nostr's censorship resistance works. It is so effective, it is even difficult for you to censor yourself!

For the same reason, editing an existing note is problematic. It only works in so far as the edit propagates to all relays the original note is now stored on.

As such, it is best to consider anything you post on Nostr to be permanent in its original state. This is actually how the rest of the internet works, too. Even if you delete something you posted on some other social media site, you have no guarantee that someone didn't take a screenshot that they can share at any time.