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- Install the latest version of iTunes on your computer.
- Plug in your device to your computer.
- In iTunes, select your device.
- In the Summary pane, click Check for Update.
- Click Download and Update.
You can check method to download latest version of iTunes from below link
http://support.apple.com/kb/HT5654
You can look into step by step instructions on updating iOS8 via iTunes on by clicking here .