tohle jsi zkoušel?
nuwi 255 bez resetovacího tlačítka se resetují takto:

1. Power the unit off.
2. Press and hold (and continue to hold) the bottom-right corner of the screen while powering the unit on.
3. Continue to hold the screen until the "Do you want to erase all user data?" prompt appears.
4. Press "yes" and the unit will reboot. This resets all your location settings and user preferences.