There's a little 1 or a little 2 in top-right corner of the display. You can flick between each using the round button on the bottom of the stalk marked ' OK || Reset '
They can be re-set independently of each other - I use '2' as a lifetime total (Avg MPG 26.6) and '1' just keeps the trip totals you are on.
FYI, if you go somewhere the trip information re-sets itself after a couple of hours if you don't re-start the engine.