Press and hold the Set button for about three seconds until the screen changes. You then need to short press the Set button or power button to scroll through the settings until you get the speed limit.
If you see settings, but not the speed limit, while you're in that settings mode, press and hold the Set button again for about 10 seconds until it changes again, this time to advanced settings.
If you don't get to any settings by pressing the Set button, press and hold the up and down buttons together and apply the above procedure.
Often, you have to do this procedure within 5 seconds of switching on the display, otherwise it only goes to the trip settings instead of the setup menu.
Whatever settings you arrive at, you can change them with the up and down buttons.
You can't do any harm by pressing any sequence of buttons, but when you get to any settings, don't change any with the up and down buttons unless you know what they are.
Some LCDs only give you a few seconds to make your mind up what you want to do before reverting back to normal mode, which is annoying because you have to keep starting again..