iOS 8.1.1 can be installed on the iPhone 4S and later, the iPad 2 and later, all models of the iPad mini, and the fifth-generation iPod touch.
As always, you can install the update via Wi-Fi by going to Settings > General > Software Update. And you can also update your device by connecting it to your desktop computer, booting up iTunes, clicking on the icon for your device in the row of icons at top left, selecting Summary in the left menu, and then clicking on Check for Update.
If you have very little free space on your device, you may not have the option of updating via Wi-Fi, and will have to either free up some space or update via iTunes.
Be sure to back up your device before installing the update.