Let’s all give AT&T a slow clap. It only took them three months after every other carrier released it but they finally have dropped Android Marshmallow on the Galaxy Note 5 for their customers. Well done.
The OTA update for Marshmallow 6.0 is huge at 1.4GB so be prepared to wait a while for the download. Once you get the update, it will take 20-30 minutes to install and your phone will reboot. As always, I highly recommend that you have all your files and photos backed up before you begin the update process.