The latest version of Apple's mobile operating system will be available a few days before the new iPhone 6s and 6s Plus start shipping on September 25.
You can upgrade your iOS device to iOS 9 on the device itself though the Settings app (General > Software Update). You can also download iOS 9 on you Mac though iTunes, and then upgrade to the new operating system with your iOS device connected to your Mac.
To get more info about the features in iOS 9:
Though Apple didn't single out El Capitan during the Tuesday event to announce a release date, it was reveal during the iPhone demo by Apple SVP Craig Federighi. An email message that Federighi showed stated the El Capitan release date as September 30.
Users can upgrade their Macs to El Capitan through the App Store. The upgrade is free. Before you upgrade, you should back up your data.
Want to learn about El Capitan’s new features We have a bunch of first look articles you can read: