I've tried several times.
First was OTA from iOS 9 beta 1 to beta 2, failed.
Then I installed the iOS 9 beta 2 firmware from iTunes, failed.
Restored to iOS 8.3, then installed the iOS 9 beta 2 firmware - finishes the firmware update, reboots - blue screen (infinite reboot).
Restored to iOS 8.3, then to iOS 8.4 beta 4, then installed the iOS 9 beta 2 firmware - it just hangs at the end of the firmware update.
I used DFU mode to restore back each time it failed.