How can I use the Trip meter in my ATV?

So I traveled about 6200km with my Linhai M150 ATV until I realized there is a meter in the screen that can measure a trip. You can reset the trip to 0km whenever you want. My friend has reset it while playing with the buttons. However, I do want to use the option but I don't how to reset the Trip to 0km. It has buttons, the one on the left switches between ODO and Trip, while the one on the right does nothing. I think my friend reset it with the right button but I don't know how. I tried pressing it once, and tried pressing and hold but it is not resetting. What should I press to reset the Trip to 0km? My friend has forgotten how