Upgrading to iOS 8.1 and Beyond
It’s important to understand that not all iOS mobile devices are compatible with iOS 8 (or later versions of the operating system). While all new iPhones, iPads, and iPod touch devices currently being sold are compatible with iOS 8.1 (or later), this operating system will only work with certain older devices, including the iPod touch (5th generation), iPhone 4s/5/5c/5s/6 and 6 Plus, as well as the iPad Air, iPad Air 2, iPad 2, iPad 3rd generation, iPad 4th generation, iPad mini, iPad mini 2, and iPad mini 3.
Upgrading a compatible iOS mobile device to iOS 8.1 (or later) is free. The easiest way to do this is to place yourself within the signal radius of a Wi-Fi hotspot and connect your device to the Internet using a Wi-Fi connection.
If you’re updating to iOS 8 from an older version of the operating system, be sure that at least 6GB of internal storage space is available within your device before initiating the update procedure. However, if you already have iOS 8 running, and you need to upgrade to iOS 8.1 (or later) less available internal storage space will be required.