Apple has released the second beta version of iOS 9 for compatible iPhone, iPad, and iPod touch devices. The new beta build is 13A4280e and can be downloaded as firmware through the Apple Developer Center, or as an over-the-air update on a device currently running the prior iOS 9 beta through Settings > Software Update.
Much like the prior version, the new build can technically be installed on any device without UDID activation, though that is strongly not recommended. Users who are curious about running iOS 9 are better off waiting for the official iOS 9 Public Beta program to launch in the coming weeks, if not being a bit more patient for the final stable release due out this fall.
It remains possible to downgrade from iOS 9 back to iOS 8 if you happen to update to the unstable beta builds and would like a return to a regular version.
iOS 9 includes a variety of new features, with a heavy emphasis on refinements and intelligent changes rather than big additions. With smart recommendations based on habits and device usage, a smarter Siri, revamped keyboard, and a new focus on battery optimization, and, for newer iPad users, an all new multi-tasking mode with split screen functionality and Picture-in-Picture hovering displays.
Separately, Apple has released a beta of Watch OS 2 (and has since pulled it for some reason), and OS X El Capitan Developer Beta 2 as well. An updated version of Xcode 7 beta 2 is also available for developers to download.