How do you replace the odometer sensor on a 1996 Ford Taurus?

There is no such thing as an "odometer sensor." There is a vehicle speed sensor that is located on the output shaft housing of the transmission. That sends info to the speedometer. From there, the speedometer and the odometer are both operated internally, so if your speedo is working, but the odometer isn't, you have a problem inside the speedo head assembly.