There are two ways to Factory Data Reset (FDR) your Chromecast:

  • Factory Data Reset your Chromecast from the Chromecast app. You will find the option to FDR under ‘Settings’ or ‘Menu’ or
  • Physically hold down the button on your Chromecast for at least 25 seconds or until the solid light begins flashing.

For the latest methods to factory reset your Chromecast or for any other troubleshooting, please see Google's FAQ page:
https://support.google.com/chromecast/answer/3210071