Very simple. Use some general WinCE shell unlocker, several ones are available on the Forum (e.g. for Mio, minishell, simple shell or similar)
Edit:
Actually, it's not that simple. I looked on the German forums.
They are very difficult to unlock. But you have to try, there is no other choice. No one, anywhere, has ever messed with that device

I'm not sure, but if the device was based on Navigon software, then you can also try an unlocker for Navigon devices. Look for one that has similar specifications (800x480).