How do I fix corrupt firmware on my iphone 8?

When I plug in my phone to my mac & try to restore the phone it goes through the motions, downloads & then gets to the end, it says "Your mac is preparing to restore the software on this Iphone" then a warning pops up & it says "The Iphone could not be restored because the firmware file was corrupt. Disconnect and reconnect, then try again" - I've tried restarting laptop & phone just keeps doing the same thing.