iOS 9.1 Beta 2 Released to Developers & Public Beta Testers
iOS 9.0.1 Released, Focused on Bug Fixes & Improvements
You can head over to Settings > General > Software Update on your iOS device to download the update.
Update: Apple has also released iOS 9.0.1 for iPhone 6S and 6S Plus as well. So when you get your iPhone 6S, be sure to update it to the latest version of iOS, with iOS 9 most likely coming with the new smartphone out of the box.