When an update for the AirPOS application has been released you'll see a notification at the bottom of the screen. If you tap on that on the POS screen as shown below your screen will refresh.
You'll see a button to install the Update. Tap on that as shown here.
Your screen will refresh and inform you that the update is being applied.
This will only take a few moments after which your POS will be setup as usual and you'll be back to the employee login screen.