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Since this iOS has been replaced by newer versions, you may not be able to download it through the traditional methods (over the air or through a regular iTunes update). On your computer, go to a site where you can download iOS 6, such as: Cult of Mac Redmond Pie IPSW Get iOS You'll need to know the type of device you're using (such as an iPad), the model (or possibly your carrier), and the iOS version you want to download: for iOS 6, the final version was iOS 6.1.6 Save the file to either a new folder or somewhere that will be easy for you to find. Download the most current version of iTunes if you don't already have it installed on your computer. Or, update your version if a newer one is available. Back up your device once you have the latest iTunes version. Or, back up your data to the iCloud. Use the Apple USB cable to pair the two, and then launch iTunes on your computer. Under the Summary tab, Option+Click (Mac) or Alt+Click (PC) on Check for Updates. It will end in .ipsw. When prompted, select Update. iTunes will install iOS 6 on your device. Allow it to install, and when it's finished, allow your device to reboot, or restart it yourself if you're prompted.
Visit a site that still offers the iOS 6 download. Find the appropriate download. Download your iOS. Update or download iTunes. Connect your device to your computer. Select your device. Open the file you downloaded.