I have a KNA-G610. Was murder trying to figure out how to update. However it was surprisingly easy once I did it.
First I downloaded the firmware update from the Kenwood site.
Created a 'garmin' folder on a SD card and extracted the contents of the update to that garmin folder on my SD card.
I then inserted the SD card into the slot on the KNA-G10 (Stereo on or off doesn't really matter)
With the stereo on I then hit the Navigation icon and the unit installed the firmware update on its own.
Updating the 'base map' is really easy....
First download the appropriate unlocked map for you. (This latest 2012.40 should be around 1.5gb)
If it's already named 'gmapprom' you're golden. If not, i.e. 'gmapsupp', just rename it to 'gmapprom'
Then set your laptop to 'show hidden files, folder and drives'
Plug in your laptop to the KNA-G10 via mini usb cable.
Power on the stereo
Hit the Navigation icon
It will then ask you what mode you want to enter. Enter 'Mass Storage Mode'
Back up all the files stored on the KNA-G10
Once done, copy and paste your recently downloaded and renamed gmapprom file over to the KNA-G10
Your laptop should ask you if you want to replace the existing file click yes
Now hit that little USB icon on your laptop and safely disconnect the KNA-G10 from your laptop.
The head unit should then reboot the nav screen back into nav mode.
.... and you're done.
Putting the map on an SD card would've probably been easier. I didn't go that route because I read some people were having issues with the maps not loading from time to time. And that putting it on the unit was much more stable. Although easier to brick your nav. Hence the reason you want to always back everything up before you start messing with it.
Hopefully this post will help save others, with the KNA-G10, from hours and hours of google searching. LOL!