How to restore or update your iPod/iPhone/iPad: After downloading the desired firmware below, you can shift-click (PC) or option-click (Mac) the Restore or Update buttons in iTunes. A file dialog will open and let you choose the downloaded ipsw-file. Note: Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it. If you don't know what device or specific device generation you got, click here.
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