From just looking at a picture of the unit, from the internet, with the top cover removed it appears as though there is a panel that is connected by three screws to the back of that plastic housing that covers all three meters? Looks like you might have to remove each of those three screws, and gently pry off that small assembly board that holds the bulbs in order to replace them? Or maybe to gain access to each bulb to replacement them?
I suggest that you try and get a Service Manual for more detailed description.
EDIT: upon further review of another close up picture (again off the internet) it appears as though it could be that the entire white/yellowish plastic housing for the bulbs may have to be removed in order to gain access to the bulbs? Look at each end of that plastic housing, there should be a screw holding each side, attaching it to the inside frame? There could be two screws on each side, one on top, one on bottom? I really can't tell since the picture that I am looking at is kinda blurred and only shows the screws on top? Anyway you just might have to remove that entire assembly on order to replace the bulbs?