If you want your RokuTV on a seperate account, then you will need to Factory Reset it (press and hold Reset button for 30+ seconds or go to Settings/System/Advanced System Settings/Factory Reset) and sign up with new email address during setup/activation.
Also, you should go to your old account at https://www.roku.com/ and "Unlink" this RokuTV from your account. (Do this after the Factory Reset and setup/activation under new account to avoid any errors).
Remember, subscriptions will no longer be shared if you unlink or create a new account for the RokuTV seperate from your other Roku devices.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102