Pressing info button repeatedly cycles through several parameters on the display at the top of the dash (not the navigation screen, in case you have that). When either avg. mph or avg. mpg are shown, holding the info button for a few seconds will reset it.
Be careful, though, because the avg. mpg readings you get from this thing are usually a few mpg too high. I have recorded both this readout and the same thing calculated by hand (miles from trip odometer divided by number of gallons filled into tank) at each fillup for three years (in my ms3), and the computer method is consistently higher (reports better gas mileage) by 2-5 mpg. After only a couple of tanks with my 2010 CX-9, anecdotally it appears to be the same.