The only way to change the email address, and therefore the Roku account, is to perform a factory reset. If your payment method is associated with the account you cannot control and you have active subscriptions paid through Roku, you'll probably have to contact Roku. If Netflix is your only subscription and you pay Netflix directly, just perform a factory reset and create a Roku account of your own. You'll have to add your channels again and authenticate with Netflix.
To perform a factory reset, go to Settings->System->Advanced system settings->Factory reset and follow the prompts. When creating a new account, make sure you go to https://roku.com/link when prompted. Beware of scams!
