iPhone needs urgent update - is it dangerous?

iPhone updates that Apple announces as urgent typically patch security vulnerabilities - usually in Safari or the system kernel. These holes can let attackers access your data, compromise your account, or even install malware. So Apple really does recommend updating quickly - the longer you wait, the higher the risk. This isn't a bug or a problem with the system itself, it's a patch for a problem that was already there.

It's dangerous not to update. The update itself to the current version is safe - developers test these releases. Maybe on older iPhone models you'll notice a speed drop after a major update, but that's a side effect, not a danger. If you've got a modern phone, just turn on automatic updates and don't think about it, or update manually when you have time - just don't put it off for too long.