Apple released watchOS 27 today for the Series 9 and later, the Ultra 2 and later and the SE 3. The big addition is Siri AI, and it is the same assistant Apple ships on the iPhone in iOS 27, so you can hold a back-and-forth conversation with it on your wrist. Workout Buddy also stops needing your iPhone nearby, and Cycle Tracking can now alert you when your cycle patterns suggest perimenopause.
Before you can install it, your paired iPhone has to be an iPhone 11 or later running iOS 27. That applies to a brand new Series 12 or Ultra 4 exactly as it does to an old watch. The Series 8, Series 7, Series 6, the second-generation SE and the original Ultra are all dropped. Keep the watch at least 50 percent charged and on its charger, then open the Watch app on the iPhone and go to General and Software Update.