You can sign out of Google account via a web browser on a desktop or iPhone, but you'll need to open the Settings app to sign out on an Android.
To change your default Google account, you'll first need to sign out of all of your Google accounts.
The other way to unregister Chromecast or another device is through the Google Home app. Follow these steps to unlink the device from your account: Open the Google Home app to the Devices tab. Select ...