You can find and download the latest firmware update package from the bottom of this website:
http://www.dension.com/product/car-handsfree-and-multimedia/dabu
The steps for the firmware update process are included.
To find out which DAB+U model version you have (DBU1GEN, DBU2GEN, DBU3GEN), please read the next section "I. About the DAB+U versions".