I had this problem but I've solved because someone wrote these instructions. I downloaded the version compatible with the model of my Ipad and then through Itunes I upgraded version IOS9 to previous IOS version 8.4.1. If you follow the steps explained with care, they will succeed and regain their iPad or iPhone device without losing what they had in them.
This is the link:https://www.reddit.com/r/ios9/comments/3l7nt8/slide_to_upgrade_freeze_iphone_5c/
Finally fixed it! You have to downgrade to 8.4.1. Outlined the steps below.
Step 1 - Backup your pictures! Even though you can't get to your phone and use it you should still be able to backup your pictures
Step 2 - Download IOS 8.4.1 for your iPhone or iPad from this site --> https://ipsw.me/8.4.1
If you are confused which file to download check the model on back of your phone and use site below to determine the version you need.
https://www.theiphonewiki.com/wiki/Models
The file was taking a while to download for my phone so I searched the filename on Google and downloaded from Mega.
Step 3 - Reboot phone into DFU mode - hold down power button and home button - when it reboots let go of power button - still hold home button
At this point it will ask you to connect your phone to iTunes - will have to connect phone to the computer.
Step 4 - Hold down the Option (Mac) or Shift (Windows) and and then click restore.
Step 5 - Browse to the 8.4.1 file you downloaded and select it. This should restore to the previous IOS version. Note: This will wipe phone.
Step 6 - Restore from backup. Hope that you backed up before attempting to update.
UPDATE : After installing 8.4.1 I was able to update to IOS9 with no issues. I was initially running IOS7 when I attempted to upgrade the first time. Hope this helps everyone out.