Apple TV plus

If you think someone else is using your Apple ID account, then changing your password would lock them out.

Change your Apple ID password - Apple Support

You can sign out on remote devices through your device list:

Check your Apple ID device list to find where you're signed in - Apple Support

If you want to contact Apple Support about this, then see here:

Contact Apple for support and service - Apple Support

Apple TV+ requires an Apple ID account with a password for that account. No way around that. If you didn’t have that, then you wouldn’t have an Apple TV+ subscription either.

Why do you think someone else is using your account, if you are not monitoring that account?

Did you get an email stating that? Consider that it might be phishing, if it makes no sense to you.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Identify legitimate emails from the App Store or iTunes Store - Apple Support