I've found this in the user manual on page 18, but it implies that it needed to be done when the unit was first switched on. Perhaps a reset to the factory defaults would allow it to be done again.
Starting the navigation device for the first time
When you switch on the navigation system for the first time, you will be asked to make the following settings:
–º your preferred language for using the software–º the unit of length for distances–º the time format–º the date format
–º the unit for temperature readings