Can Nostr Events be Manipulated?

If you haven’t been following along, I have a couple of nostr posts at this point. I’ve been trying to further understand nostr by deep diving the protocol. At this point my thought is how mutable are nostr messages (events), I understand that during broadcast the relay verifies the signature, but then they need to store these events in some centralized database, right? Could a rogue relay for example accept your event, then alter it at a later time?