Recently i just tried to upgrade my 2 devices :
First, iPhone 5s from iOS 7 to iOS 9 using Wi-Fi.
Second, iPad 2 from iOS 8 to iOS 9 using Wi-Fi.
Upgrade of the iPad went successful.
But on iPhone upgrade, i was stuck in “Slide to Upgrade” step.
I could not do anything. I slided thousand times. Nothing happened.