Q: I have 7.2.1 IOS, I want to update it to 10, will my contacts, photos and apps be erased ??

If your device is currently running iOS 7 there is a strong likelihood that it wasn't able to update to iOS 8 or 9, and won't be able to update to iOS 10 either. See http://www.everyi.com/by-capability/maximum-supported-ios-version-for-ipod-iphon e-ipad.html to make sure.

Upgrading iOS shouldn't erase contacts, photos, or apps, but this information should really be kept in an iTunes library, iOS device backup, or other location on your computer as appropriate, ready for the occasion when your device is lost, stolen, damaged, or needs restoring so that you can restore data to it or a replacement. See Recover your iTunes library from your iPod or iOS device if needed.

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