You might think that in order to reset Google Home and return it to factory settings you simply say: "Okay Google, factory reset." Actually, it's a lot easier that. (Also see how to factory reset Google Nest Hub.)
As a heads-up, should you give Google Home that request it won't know how to handle it.
Instead you must press and hold the microphone button on the device's rear for 15 seconds.
It's impossible to accidentally reset Google Home using this method, given that you must hold down the button for such a long time. Google Home also gives you an audible warning that you are about to reset the device, and you'll see a countdown on Google Home's surface as each LED lights up one by one to form a complete circle.
Once the circle is complete, Google Home will reset itself and reboot.
To reconnect to Google Home you follow the same procedure as you did the first time you used it. So, install the Google Home app, let it search for and connect to the device, then enter details such as the room it's in and your Wi-Fi network details, log into your Google account and follow the prompts to configure the device.
Also see: How to reboot Google Home