To update your device use TomTom home. It automatically checks for updates when you connect. You can also use the latest map guarantee (go to File and it is in the drop down list) to check for map updates. And lastly, you can check for TomTom Home updates by clicking under Help in TomTom Home. There was a new release today.
Another member had the same problem and I found this from another thread in the forum:
Please rest assured that your promotion code for a map update is not invalid. The error you are experiencing is because our map update system no longer requires a promotion code. If your unit does not have the most current map, it will appear as an update when you connect your Go 920 to TomTom HOME. Alternately, you can access it by selecting Latest Map in the upper corner. If you have not yet visited this program, it can be downloaded at:
www.tomtom.com/home