The update is available through Apple's over-the-air updating mechanism on iOS devices and it can also be downloaded through the Apple Developer Center.
Prior to the launch of the golden master, Apple seeded five betas of iOS 9 to developers and public beta testers, with the fifth and final beta released on August 6.
iOS 9 focuses on intelligence and proactivity, allowing iOS devices to learn user habits and act on that information, providing recommendations to places we might like, apps we might like to use, and more. Siri is smarter with iOS 9, able to create contextual reminders and offer up more information through improved search options.
Built-in apps like Maps, Notes, and Mail have been improved in iOS 9, and Passbook has been renamed Wallet. iOS 9 introduces split-screen multitasking for the iPad for the first time, and there are some significant under-the-hood improvements that will result in more battery life, more storage space through smaller app install sizes, and enhanced performance.