Preparing your device for the upgrade
Before committing to the upgrade there are steps you should take before doing so. The first is to back up the data on it. You can do this in one of two ways: either by connecting your iOS device to your computer and backing it up to iTunes, or by backing it up to iCloud. In this case, I'd encourage you to choose the first option. If it comes time to restore your data and apps, you'll find that doing so from iTunes is a lot faster than from iCloud. Let's start with the steps for backing up to your computer.
Back up via your computer
Launch iTunes on your Mac and, if you haven't enabled Wi-Fi sync, string your device's syncing cable between it and your iOS device. Select your device in iTunes, choose the Summary tab, and in the Backups area of the resulting window click on Back Up Now. If there are apps on your device that aren't in your iTunes library, a window will appear asking if you'd like to back them up. The choice is yours, as you can always re-download any you're missing from the iTunes store. iTunes will now set about backing up any apps on the device that aren't in your iTunes library as well as other data the device contains.
This would also be a good time to back up any purchased content on the device. To do that, choose File > Devices > Transfer Purchases From [name of device]. Any purchased media on the device will be backed up to iTunes.
Back up to iCloud
Alternatively, you can wirelessly back up your device to iCloud. You can initiate this in two ways. The first is to access the Summary tab in iTunes and in the Backups area enable the iCloud option (as seen in the image above). The other is to choose to back up to iCloud from the device itself.
You do this by going to Settings > iCloud > Storage & Backup and in the resulting screen enabling the iCloud Backup option. An alert appears informing you that your data will now be backed up to the cloud. Tap OK to allow that to happen.
Turn off Find My iPhone/iPad/iPod
If you've enabled iCloud's Find My [Device] feature to help you track it should it be lost or stolen, you'll have to turn it off before you can upgrade to iOS 8. This is a security precaution introduced with iOS 7 to help prevent bad guys from wiping your device after stealing it.
To do this, go to Settings > iCloud > Find My [Device]. Switch the toggle to Off and you'll be prompted for your Apple ID password. (This is not the security code you may have set up for the device but rather the password you use with iCloud and with the various online Apple stores.) Enter that password, tap OK, and the feature will be switched off, allowing you to update your device.
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