Download the newest firmware image from this page.
Make sure the sensor is on a network with a DHCP server and has gotten a DHCP lease.
Open a web browser and navigate to the sensor hostname: http://os1-XXXXXXXXXXXX.local
Using the browse button, select the firmware image you downloaded and click the update button. The sensor initiates its 5 minute update sequence, displays a success message, and automatically reboots at the end: