The NX6 Handheld has the ability to receive over-the-air firmware updates, these are referred to as Wireless Upgrades. These updates are in addition to the NX App updates, and pertain to the firmware of the physical NX6 device.
To manually perform a Wireless Upgrade, you'll need to take the NX6 out of Kiosk Mode. Follow those instructions here: Taking an NX Device in and out of Kiosk Mode
Once out of Kiosk Mode, you'll be able to navigate to the OS Homepage. It will look something like this:
Select Settings (grey gear icon) from the available Apps
Scroll down to About Device, it's at the bottom, and select it.
Locate Wireless Upgrade, and select it.
You'll now see the Wireless Upgrade page. If there are updates available, you'll be able to install them here.
Wireless Upgrades take some time, and require a stable internet connection. Once the Wireless Upgrade has completed, you'll be prompted to reboot your NX6. Once it's rebooted, your NX6 will be running the latest OS software.
The last step is to place the NX6 back into Kiosk Mode. You can follow those instructions here: Taking an NX Device in and out of Kiosk Mode