Tap Settings then tap General then tap Software Update.
Scroll down and tap Download and Install. Follow any prompts.
You can also update through iTunes on your computer. I'm not sure how.
Its important to do a backup before installing update.
On Wi-Fi tap Settings, scroll down and tap iCloud.
Scroll down, tap Backup, make sure that iCloud is turned on.
Tap Back Up Now.
Make sure backup is finished: Tap Settings>iCloud>Storage>Manage Storage, then select your device.
The backup should appear in the details with the time and backup size.
Make sure your battery is charged up before you start or I think you can have it plugged in and charging while updating. I'm not 100% sure about that but I have done it with no problems.