On the PUI: Press the gear icon key on your phone
Then select the menu item "System information".
Right-click on the link to copy the link to your clipboard
Open the phone web interface, and navigate to the menu item "Software update."
Insert the copied link into the text field and press load.
You can also update the firmware of Snom phones by means of auto provisioning. This allows you to update multiple phones simultaneously and with significantly less effort. We explain how this works here: Firmware Update via Provisioning