3uTools is the most powerful third-party iOS flashing tool available, but it cannot perform miracles. For devices older than the iPhone X, you have genuine freedom to restore any unsigned IPSW you can find. For newer devices, without saved SHSH blobs, you are locked into Apple’s signing window.
Open 3uTools on your computer and connect your iOS device via a lightning/USB-C cable. Ensure the program recognizes your device. Navigate to Smart Flash Click on the Smart Flash how to restore unsigned ipsw with 3utools