I played around with it some more this afternoon, and it's definitely not in any of the normal menus or settings.
The reason why it's not documented anywhere in the user manual is because the change needs to be done from the diagnostics / service menu. To access the service manual, you need hold the Display Mode button down and then turn the tractor on (but you don't need to start it though) while holding the button in. Once the service menu appears, scroll to option "J" and select it (by holding down the Display Mode button); from there you can either select kph or mph (once selected - hold button and it "OK" will appear at the bottom of the screen.)
I had to do a bit of thinking to figure this out!