Installing unsigned IPSW files typically requires the device to be in DFU mode and relies on the SHSH blobs being saved for that specific iOS version for your device. Without valid blobs, the installation will fail even with 3uTools. How to Install an Unsigned IPSW with 3uTools Apple stops signing older iOS versions shortly after releasing a new one. Once unsigned, iTunes/Finder will refuse to restore them. However, 3uTools offers a feature called "Pro Flash" (Retain User's Data) or "Easy Flash" (iTunes Flash) that can sometimes bypass this using saved SHSH blobs.
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