I managed to update the mileage on my odometer after replacing one of the gears that drive it. You need to take the speedo to bits to update your mileage. I can't remember how the 1990 speedo differs to the earlier ones so perhaps someone can advise on how to dismantle it. Anyway, you need to disconnect the stepper motor from the mileage counter by removing a couple of screws that go through the motor. Then you can manually wind the counter up using the least significant digit on the counter. It could take you half an hour to add 10,000 miles though!

If you know a bit about electronics and have a signal generator you can connect it to the speedo to simulate the car being driven. If you do this then make sure that you don't exceed the maximum clock frequency of the odometer circuit.
Ignore this last paragraph if you don't understand. It is much easier and faster to correct the mileage manually.

Cheers
Will

1986 740 GL 235k mi.