hey hsnmz, love your choice of words,, lol @ "My steermatic just turned 6000 Km youg :)". Thats how most old folks in the states mention their age, lol!..
any ways, getting back to your question, use to knob to switch to either Trip A or B, and keep it depressed for 3 seconds or so, and it shall automatically reset to 0. best of luck!