Beta Upgrade iOS 9.X

Go into iTunes with the phone connected to your pc/iMac



Back up your device

https://developer.apple.com/ios/download/


My device is model a1533 (listed on back of iphone)


Select your model and it starts the download:

Hold Alt/Option (on a Mac - it's Shift on a PC) and click the Restore iPhone button on the device's Summary tab (next to Check for Update)