Restoring an unsigned IPSW is technically impossible for the average user because Apple must provide a digital signature (SHSH blob) to authorize the installation. Once Apple "unsigns" a firmware version, their servers will no longer provide this authorization, causing standard tools like iTunes and Finder to block the process.
Using an unsigned IPSW restore tool requires some technical expertise and caution. Here's a general outline of the steps: unsigned ipsw restore tool free
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