WatchOS 2.0 was supposed to be released last week alongside iOS 9, but a showstopping "bug" held the release up. Apple has apparently fixed it because the final version of WatchOS 2.0 is beginning to roll out to all editions of the Apple Watch today. To install the update, open the Apple Watch app, go to General, go to Software Update, and then follow the instructions.
Developers who installed the earlier "Golden Master" build of WatchOS 2 should also check for an update my 42mm Apple Watch Sport is getting a 12.4MB patch that should include the fixes in the general release. It should also be noted that installing WatchOS 2 requires your iPhone to be running iOS 9.
WatchOS 2 makes a few additions to the core operating system, but the bigger news is that Apple is opening more hardware access up to third-party developers. They can make apps with more complex animations and access hardware like the Taptic Engine, Digital Crown, and other sensors. These apps will also be able to run without an immediate connection to your phone, something only first-party apps could do before.