To factory resset your third-gen Chromecast, follow these steps:
Open the Google Home app.
Select your Chromecast device > Settings (cog icon)
More (three vertical dots icon) > Factory reset > Factory reset.
Alternatively, you can reset the Chromecast directly while it’s plugged into your TV.
Hold down the button on the side of the Chromecast.
Wait for the LED to blink orange.
When the LED is white, release the button.