Regardless of whether you choose to update iOS over the air or via iTunes, it should only update the operating system and Apple's default apps. Any other apps and data on your device will be preserved in their current states.
Starting over with iTunes
If you're the kind of person who uses a major iOS update as an excuse to start fresh — with a device that behaves as if it just came out of the box — you have the option to completely wipe the apps and data from your iPhone, iPad, or iPod touch; install iOS 8; and then operate it as if new (or restore it from a backup). One avenue for doing this is via iTunes.
To proceed, in iTunes' Summary tab, instead of clicking Check for Updates, click the Restore button (Restore iPad... or Restore iPhone..., for example). If Find My iPhone/iPad/iPod is switched on you'll be asked to disable it (as described earlier). If you have purchased media on the device that hasn't been backed up to iTunes, you'll be prompted to do that. You'll then be asked if you're really sure that you want to restore the device to its factory settings. If you're certain you have a backup, go right ahead.
Again, iTunes will download the version of iOS 8 appropriate for your device and then install it. During the installation you'll see the Apple logo on the face of the device and a progress bar, giving you a general idea of how long it will take to install iOS.
iTunes will display a series of messages, telling you it's waiting for your device to get ready, verifying that the device can work with that version of iOS, and then telling you that it's restoring your device.
When iOS is finally installed, iTunes offers two choices. You can set up the device as a new one — meaning your device will be set up as if you've just pulled it from the box. Or you can enable the Restore from this backup option and then, from the pop-up menu, choose a backup you've created. (Ideally the latest backup you've performed.)
Set up as a new device
Choose Set up as new iPhone/iPad/iPod and click Continue and you'll be walked through the process of setting up the device just as you did when you first got it. The first thing you'll see is your device welcoming you with a cheery "Hello" (or a greeting in another language). Do as the screen tells you and slide to the right to get started.
This begins a process where you're presented with a variety of setup options. They include choosing a Wi-Fi network, agreeing to Apple's terms and conditions, choosing to use location services, entering your Apple ID (or creating one if you don't have such a thing), choosing to use iCloud, enabling the Find My feature, selecting identities to use with iMessage and FaceTime, choosing and confirming a passcode, allowing your device to use iCloud Keychain, choosing to use Siri, and deciding whether or not you want to send diagnostic and analytics data to Apple and third-party developers. If you're unsure about what a particular option does or means, there's generally a bit of text that explains it (or a link to more information). A Back button is available on every screen so if you've made a mistake, just retreat through your previous steps and correct the issue.
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