Apple released a fix for this issue in iOS9.0.1. Yay!
We're seeing a lot of reports on Twitter and the Apple forums about folks being stuck at "Slide to Upgrade" after updating to iOS9. Some people seem to be fixing it by downgrading to iOS8.4 and then upgrading again to iOS9.
What if I Can't Downgrade due to Find my iPhone?
When trying to install iOS8.4, you may be told that Find my iPhone needs to be turned off for your iPhone. In this case, you need to put your device into DFU mode.
If you're still on the frozen slide to upgrade screen, press/hold the home and power buttons together to force the phone to reboot.
When you get back to the slide to upgrade, slide and do your passcode, but press nothing else after that.
Make sure you are plugged in to your computer via USB.
Hold the power button to bring up the slide to turn off, and slide to turn off.
When the iPhone is powered down, press and hold both buttons for 10 seconds. Then let go of only the power button, still only holding the home button.
Keep holding the home button until you see the lovely connect to iTunes screen.
Now you should be able to press that "Restore iPhone" to reinstall iOS. Don't forget to hold down Option (OS X) or Shift (Windows) to select which iOS install you want!
I Upgraded, But Restoring My Backup Brings the Problem Back!
We're working on an update to Decipher Backup Repair for this problem. Yay!
Others still seem to be stuck in the situation that a fresh install of iOS9 works, but restoring the data from the pre-iOS9 backup brings the problem back. If there's one thing I love, it's an iPhone mystery involving backups! If you're in that situation where restoring your backup causes your iPhone, iPad, or iPod touch gets stuck after restoring your backup, reach out to us through our support form. We definitely want to help figure out what is in these backups so we can fix them and get people up and running ASAP.