Connect your phone to itunes, in thrme summary tab displaying your phone informations, there you will see the most recent available firmware for your device. If it doesn't correspond to what u need, then google to download the firmware you need, if it correspond to what you need, then clic "upgrade" a notification will popup asking you if you want to download the latest firmware and install it right away or if you just want to download the firmware and install it later, chose download firmware and install later. The itunes will start downloading the latest firmware and when the download is finish you will found it at that location i mentioned in my previous post
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