how to change my email for apple TV

The Apple ID account is for all of your Apple devices and services; not just Apple TV. Users are generally advised to bring all services together on one account, which should be perfect for most users.

• If you want to change the email for your current Apple ID account, while keeping all the other account properties (registered devices, subscriptions, purchases, etc.), then do as described here: Change your Apple ID - Apple Support. After the change, you may need to sign out and sign in again on your devices.
• If you want a new Apple ID account for use with just Apple TV, then you can create a new account, but you won’t be able to transfer existing properties (subscriptions, purchases, etc.) to that new account. How to create a new Apple ID - Apple Support. It is like starting from scratch.