Along with any kind of software release or update, one thing is always necessary: storage space. It doesn’t matter how big an update is, you’ll always need more space than that to be able to download it. It is no different for iOS 8. For the newest version of Apple’s software, you’ll need a whopping 4.6GB of free space in your iOS device.
Anyway, before updating to iOS 8, make sure to clean up your device and remove any useless stuff, apps, photos that you don’t need any more…
Have you downloaded iOS 8 on your iPhone ?