So, having done my own unit today, I can add some further guidance and tips.
The kenwood site has all the latest firmware updates, go to
www.kenwood-electronics.co.uk and you can search for DNX525DAB, this will give you a specs page, click on the firmware tab for links to download the firmware for system, Bluetooth and DAB.
The download for each is a zip file, you have to extract the contents. Copy the contents of the extracted folder (ie the individual files) onto a USB stick, do not try and use an SD card. I spent two hours trying every combination of files on an SD card today, the kenwood unit wouldn't see any updates, I put it on a USB stick and it recognised it straight away!!!!
You need to go to main menu>set up>software, if the USB is plugged in then it should give you the "update" button, click on this and it should install the updates as needed.
Hope this helps. I have yet to try any map update, will be looking at this next.