You should be able to re-set total distance (trip odometer) by going to the navigation page (the one with the compass) and clicking the enter button (bottom left) then select 'Reset Trip' and enter again. This will reset the odometer.
If that's not what you mean by overall distance, what do you mean, as I've never seen another overall distance on mine. If there's a 'total covered by unit since manufacture', I'd love to see how far mine's gone.
BTW, in your profile you say that you've been told the yellow fellow is rubbish. I'd disagree. It's basic, but it should be as accurate as more expensive units, it's very robust, and being yellow means it's easier to find under the bush you've just dropped it in.