1. You need to go to http://www.netflix.com/ and log in.
2. Look at the top right corner and click on "Your Account".
3. Look under settings and click on "Manage devices and computers".
4. Then click the button "Disconnect Devices".
Then you should be logged out of Netflix on your Xbox. Then you'll get a message saying "this device has been deactivated" and it will ask you to sign in again.