Unlatch the front panel below the jackpot key and slide board out the front of machine.
To expand on this, open the main door/hatch, observe that there's a metal pin under where the armrest had been on the inside of the machine. When you lift that pin, you can remove the front panel by your legs (watch your feet! If that panel falls off when you lift the pin you could really damage your toes). With the panel removed, you can easily access the MPU tray on the left side of the machine.
Note: always ensure that the power is OFF before removing or inserting the MPU board.
Also, the hopper bowl is shaped in such a way that it's possible to remove the MPU board without removing that front panel. It's a tight fit, but you can do it.
Finally, be sure to put the front panel back in place for game play or the machine will detect that a door is open and it won't work.