Should be a very small knob sticking down vertically out of the bottom edge of the instrument panel, close to the steering column on the console side. That is on the end of a short speedo-type mechanical cable connected to the odometer. Turning the knob should allow you to reset the odometer digits to zero (you may need to push upward on the knob while turning it).
It's not uncommon for this mechanism to fail, however, due to stripped gears at the odometer end of the cable or in the odometer. Parts are available, albeit rather expensive, from vendors such as Long Island Corvette
http://www.licorvette.com
Art
Resetting The Trip Odometer On My 64
Linear Mode
