Nexus 9 is now receiving Nougat update on T-Mobile network. The latest update brings Android 7.1.1 Nougat as well as Android security patches for the month of December for HTC-manufactured Google Nexus 9. The update is just 52MB in size.
In order to update Nexus 9 tablet, make sure that the device has 50% or higher battery life. Needless to say, a data connection (preferably WiFi) is required, and there should be sufficient free memory available on the unit as well.