Switchting from miles to km/h?

Call the service technicians at the Hummer dealer service shop. They have to be schooled in all the systems that are built into the Hummer brand in order to be proficient in fixing problems.
The factory may also have a hand in the Odo measurement when the vehicle is built. For example if there is a production run for Cananda, then all vehicles in that run would have the Programming of the Odo measurement set up in Km. It is I'm sure as simple as a programming code that is set into the vehicles on board computer.
Same way mixture and engine timing can be altered.