This procedure may wipe your data and apps. Meaning, first check the firmware build number of your device and then flash the proper OTA file.
You will need to update your firmware gradually. Hence, you need to flash them either via stock recovery by choosing apply updates from SD Card or via ADB sideload method.
How to Restore AT&T Note 4 to stock firmware using Odin file? How to Install AT&T Note 4 N910A Android 6.0.1 Marshmallow N910AUU2EPG7 OTA Update?