Open the Google One application then go to the hamburger menu in the top left hand of the screen, tap on it then tap on Settings. Scroll down after you tap on Settings and you'll see the option of Cancel membership.
All I'm seeing is an option to cancel google one. I just want to kill Gemini dead, burn it, and scatter it's ashes to the four winds. Isn't there a way to cancel Gemini and leave Google One intact?