In addition to releasing the Gold Master of iOS 9, Apple has also released the first beta of iOS 9.1, expected to ship later this year. Developers can grab it from the iOS Developer Resource Center right now.
Not much is known about iOS 9.1 at this time. It's expected to arrive for iPhone and iPad later this year, perhaps alongside the release of the iPad Pro. For now, interested developers should download the beta on a secondary device, rather than the iPhone or iPad they use every day.