HomePod Restore is a tool created by UnbendableStraw that can be used to restore patched IPSWs to the HomePod 1st Gen. It requires the use of a tool known as HomePWN , HomePWN provides a USB connection to the HomePod which can be used to jailbreak it or restore it.
Patched IPSW
The patched IPSWs required by HomePod Restore are created using a tool called HomePod Stuff. HomePod Stuff uses a signed tvOS IPSW, audioOS OTA update file and tvOS IPSW decryption keys to create a restorable IPSW for the HomePod 1st Gen.