Hello everyone,
Resetting Chromecast 1st Generation:-
It is quite easy to reset Google Chromecast by following these two different methods.

Method 1: Using Google Home App

Make sure you have updated Google Home App.
Open Google Home App and Tap on Chromecast devices.
Tap Settings at the top right corner.
Now, Tap More or three-dot icon > Factory reset > Factory reset.
That’s it

Method 2: From the Chromecast itself

Turn on the TV where Chromecast is connected.
Now, hold the button present on the rear side for few seconds until a solid LED light starts flashing.
The TV will go blank and Chromecast will reboot

Thank you.

For more details:-

Chromecast Set Up
Chromecast Not Working