How to fix the iOS 9 'Slide to Upgrade' bug using iTunes
Since the release of iOS 9 this week, a number of users have reported an issue in which their devices are rendered useless by an unresponsive "Slide to Upgrade" screen that appears after installing the new operating system.

Apple responded to customer concerns on Friday with a simple fix posted to its Product Support website. Because it requires users have access to a computer, the workaround is a bit clunky, but is proven to be effective.
First, connect an affected iOS device to a computer running iTunes via USB. Select your powered-on device from the device list located in the top left corner of iTunes, represented by its respective icon.
If the device is not recognized, conduct a force restart operation by pressing and holding the sleep/wake and home buttons for at least 10 seconds. A successful force restart will result in an Apple logo being displayed onscreen.

Next, select Restore Backup... in the Backups box and choose a recent backup file. Users who do not have a backup stored will need to manually create one by clicking the Back Up Now button, then restore from that file.
Apple is expected to issue a permanent fix in a later iOS update.

Apple responded to customer concerns on Friday with a simple fix posted to its Product Support website. Because it requires users have access to a computer, the workaround is a bit clunky, but is proven to be effective.
First, connect an affected iOS device to a computer running iTunes via USB. Select your powered-on device from the device list located in the top left corner of iTunes, represented by its respective icon.
If the device is not recognized, conduct a force restart operation by pressing and holding the sleep/wake and home buttons for at least 10 seconds. A successful force restart will result in an Apple logo being displayed onscreen.

Next, select Restore Backup... in the Backups box and choose a recent backup file. Users who do not have a backup stored will need to manually create one by clicking the Back Up Now button, then restore from that file.
Apple is expected to issue a permanent fix in a later iOS update.
Comments
Upgraded 6 iOS devices, including iPhones, iPads, and iPod Touches, and never came across this.
Mine was bricked by the update from USB. After many tries, I was able to wipe and restore as new. Then restore from backup. Seems iTunes was trying to connect to Apple to verify the phone and failing to do so. Thus the new OS was never installed and bricked the phone for several hours....
I upgraded my Retina iPad (iPad 3) without a problem, but it took a VERY long time, like somewhere around an hour. I was actually worried that it was in some kind of reboot loop.
What the big rush to waste time upgrading to iOS9 ?
At iOS9 V2+ I'm sure the upgrade will proceed very fast and correctly !
My phone's photos show up in Photo, but the screen remains locked at "slide to upgrade%u201D.
I updated 1 phone and two iPads without issue. However, I always tell iTunes to download the install file separately first. Then I do the install after that has finished. Not sure if this matters one bit, but I have never had OS install issues with any iOS device.
I worked for Apple on iOS customer support a few years back, and doing a restore from backup was the standard procedure after the few easy fixes had been attempted. I was astonished how many people had phones that had no backup, and had never been connected to a computer/iTunes for updates. They didn't even use iCloud backup.
What the big rush to waste time upgrading to iOS9 ?
At iOS9 V2+ I'm sure the upgrade will proceed very fast and correctly !
ummm, how about an hour longer runtime. For me that is a BIG deal. I've updated my iPhone 6, iPhone 5, iPad 2, iPad 4, iPad Air2 and iPad mini all without a glitch and I do get easily an hour more runtime. Very happy:)
What the big rush to waste time upgrading to iOS9 ?
At iOS9 V2+ I'm sure the upgrade will proceed very fast and correctly !
ummm, how about an hour longer runtime. For me that is a BIG deal. I've updated my iPhone 6, iPhone 5, iPad 2, iPad 4, iPad Air2 and iPad mini all without a glitch and I do get easily an hour more runtime. Very happy:)
Glad to hear your upgrade went OK.
As for an hour more of run time, since I go 2 to 3 days per charge on my iPhone, I wouldn't notice the difference.
I use my iPhone quite a bit, but only for short time periods if not travelling. Weather, stock info and a Email peak.
When travelling I use my iPhone for most of my computing activities, but only morning and evening when on WiFi.
I do most of my reading on our iPad and my Mac desktop; even a large screen iPhone+ is a bit small for extended reading.
when I do this it comes up saying that I have to turn off find my phone app
but how??
This is why I never upgrade the first week of any OS update. Apple used to be fantastic at making sure OS updates ran smoothly and worked, but I don't see that attention to detail anymore and I've had problems even with the minor iOS updates. 8.4.1 looked to me like it downloaded and installed itself twice, even though it should have recognized that it was already installed the second time. Messaging during the update has also been pretty bad and frequently illogical. Luckily, I was patient and walked away when it looked to me like it was hung and it finally ran. Once the update downloads, I don't see why server traffic should still be an issue, but apparently it is.
Whether this is a problem with the update itself or a problem with Apple's servers, is beyond me, but it should just work. If I want stuff not to work, I'll return to Microsoft's platforms.
And ever since the last iTunes update, I don't have progress indicators anymore within iTunes.
Right now, my phone claims it has downloaded the iOS 9 update. If I plug it in via USB, does it use that update or does it download another one? It always seems to me that it downloads another copy, otherwise it should start installing right away and it doesn't seem to. And if I do plug it in, it's going to tell me to transfer purchases from the phone to the Mac first, but then it doesn't automatically run the update. I forget what happens next, but it's not smooth and the on-screen messaging is pretty poor. For a company that used to get this kind of stuff so right, it's shocking to me that they frequently now get it so wrong.
Really? Restoring the system from a backup is considered a fix? That something I would expect on the Microsoft support pages.
Really? You didn't know it's been this way since the beginning of iOS? That's something I would expect a real iPhone users to know.
i update 4 devices on Day 1, every single time. no issues. so here goes your hypothesis that Apple software sucks now.
i update 4 devices on Day 1, every single time. no issues. so here goes your hypothesis that Apple software sucks now.
That's like saying that you drove a Toyota that didn't have a sudden acceleration problem and therefore there's nothing wrong with Toyotas. Just because you didn't have the problem doesn't mean it doesn't exist. But there's plenty of evidence that many people (certainly not all) have had problems with Apple iOS upgrades, especially this latest one.
I was running a jailbroken iOS 7.04.
When I upgraded to iOS9 straight from 7.04, it got stuck on the slide problem. Multiple attempts with reboots, etc, and no good.
I was able to upgrade to iOS 9 with a clean install, but once installed, if I tried to restore, I got the slide problem.
Solution: I upgraded to 8.4.1 first. Afterwards, no problem.
If you're trying to upgrade/restore from a different iOS than 8.4.1, first upgrade to iOS 8.4.1 then try iOS 9.