watchOS 2 requires iOS 9 and can be downloaded over-the-air through the Apple Watch app on the iPhone by going to General --> Software Update.
Today's golden master release follows five developer betas of watchOS 2, which were seeded to developers from June 8 to August 6, when the fifth and final beta was released.
watchOS 2 is the first major update to watchOS, the software that runs on the Apple Watch. watchOS 2 introduces several new features developers can use to improve their apps, including more efficient native apps that run on the watch itself instead of the iPhone. The new software also allows developers to access the Taptic Engine and sensors like the heart rate monitor, microphone, and accelerometer.
Thanks to the new tools for developers, Apple Watch apps will be more complex, full-featured, and faster. Other features in watchOS 2 include photo and time-lapse watch faces, third-party Complications, a longer screen activation time, Time Travel, and a Nightstand mode that's activated when the Apple Watch is charging.