Nothing. They cannot be manipulated independently. I'm sure the display manufacturer intended them to be but it is not an option on the M5 display with Aventon.
One of the old Pace manuals (350?) has Trip Odo reset as press & hold up arrow button for 5 seconds. Don't know if that is still applicable. Several other ebikes I've seen (mine included) handle Trip Odo reset by a simultaneous push-and-hold of 2 different buttons.