On my 6500 I had to do AC (All Clear) first, by holding down the 'B' button for 5 seconds, then the initial menu comes up, the first one is KM/MPH which you toggle with button 'A'. After that you can go thru the rest of the menu or just hold button 'B' for a couple secs to save and leave that menu.
Note that this will clear out all your memory storage.