Keep the trip button pressed down for about 5 secs, while you press power once (not to start the engine, just to turn the igniton on). It puts you in a test mode, which you can cycle through using the trip button. It wont stay on the MPH when you turn the engine off and back on again,so if you want it all the time looks like you have to hold trip every time you start the engine.
There are some other interseting things in the test menu - not sure if you can alter anything or if they are just for information??