According to the Freightliner Dash Gauges Guide, a short push of the switch will toggle between the tripometer and odometer.
A long push/hold of the switch when in the tripometer mode will reset the tripometer.
It also states that a long hold of 3 seconds or more will toggle between miles and kilometers when in the odometer mode.
Hmmmmm... I have never tried to see if it works, but truthfully I never knew about it. But like you, I don't do metric.
Isn't it great to learn about things you never knew you had and probably will never use.