Q: Slide to upgrade

Have you tried resetting the device: Hold down the Home and Power buttons at the same time and continue to hold them down until the Apple appears (up to 30 seconds). Once the Apple displays, the reset is in progress and the Slide to Unlock screen should display shortly.

Cheers,

GB

What iOS are you running on your device? Did it start going to that Slide to Upgrade screen as a result of an iOS update or as a result of something else that has changed on your device?

Here is a support article that is for an older version of iOS 9, but may help (you would have to update to iOS 9.3.1 at this point, but the instructions may help to get you to that point:

If your iOS 9 device is stuck on Slide to Upgrade - Apple Support

But it would help to know what the mitigating factor was for this situation.

Cheers,

GB

Try the troubleshooting steps in the article that I posted.

Best of luck,

GB