The "Slide to Upgrade" bug is a glitch that is affecting users who are trying to upgrade their Apple devices to the newly released IOS 9 mobile operating system. 
The problem occurs after users have installed the update. Their device becomes stuck on a screen that prompts them to "Slide to Upgrade", but does not allow them to complete the task. Instead, the screen simply freezes and the user is unable to slide as instructed.
According to Apple, to solve the problem, users have to connect their device to a computer and restore it from an iTunes backup. You can see the steps that they recommend following to correct the problem here:
If you prefer, you can also watch the following video that walks you step by step through the process of fixing the problem: