Following Sprint, AT&T has also started pushing out the Android 7.0 Nougat update to it's Galaxy Note 5 units. It's an over-the-air update so will roll out gradually and will take time to reach every unit. The update is 1.44GB in size so make sure your device is charged full and connected to decent internet connection.
Arriving in with build number NRD90M.N920AUCU4EQC6. Aside from this update also includes android security patch for the month of March.
Head over to setting's menu of your phone to check whether the update is available for your device or not.