Online

In the webinterface of the Maxwell 10 go to: 

Settings - System - Firmware update

Data server URL = 

http://profile.gigaset.net/device/temp/Maxwell/80_1_1.2.20.1_maxwell_10-user_firmware_enc.zip

Click on Update firmware and the latest Firmware that is online will be downloaded.

Manual

First download the Firmware on your PC and unpack the zip file. You will get a .txt file with the Open Source License text and an encrypted zip file which is your firmware image.

In the webinterface of the Maxwell 10 go to:

Settings - System - Firmware update

Click on "Choose File" and select the Firmware file.

Click on "Load" to start the upgrade.