Download the IoTT Stick software from the Github page. Make sure to download the latest version.

Then unzip the install file on your hard drive and follow the instructions in the Readme.txt

If you are familiar with the Arduino IDE, you can also compile your own software. 

Download the sketchbook directory from the repository. You can choose between the master branch, which holds the current released version, and the development version, which has everything I am currently working on, but may have bugs or experimental functions.

Hint: Use the Save to File function in the Node configuration page to copy all your setup data from your IoTT Stick to a local file. The software upgrade will delete all user data on the Stick, but with everything saved in a local file, you can simply reload it after successful installation of the new software version.