I'm not sure how to change it, but i know how to get into it. Push the menu button on the back of the computer, then push mode 5 times and it should come up with km/h flashing and then mp/h underneath it. I've had a go changing it to mp/h but i can't seem how to work it out. It says on the instruction sheet that you push mode to change it, but it didn't work for me...

Happy b'day by the way