You must found out the source of your problem here. It can be one of two things:
Red Outside Gear
While holding onto the gold inside number gear, turn the red gear. If the shaft is turning with the gear, then your problem is below. If the shaft is not turning with the gear, then you'll have to replace the gear. Count the teeth on the gear and purchase a new one. Install using a bit of epoxy.
Gold Inside Number Gear
If you've done the above and found the red gear to be in working order, then this is your problem. You must pull the rod out of the gear and odometer numbers and burr the end of the rod with a knife or other such object. You can burr too much and then your rod will be useless. Be careful. Replace the rod and now it should fit tightly within the gold gear. Spin the red gear on the end and make sure everything is in working order.
Replace the odometer unit and follow the above directions in reverse order to put your cluster back together again. If you've done everything correctly and fixed the problem, then you should have a working odometer. When reattaching the speedometer needle, don't put it on at 0 (mph/kmh), but before it. There should be a little tick mark at the bottom near the reset pin. Line it up with that to get the correct spring tension.
This is, perhaps, the most daunting of the tasks. This is the easiest and, maybe, safest way of resetting the mileage on your odometer. It should be known that you must note this alteration when selling your car.