Unplug the unit from power.
Remove decorative aluminum band from motor enclosure by removing single screw at rear of mixer.
Remove screws (now exposed) which secure the top half of the motor enclosure.
Using a 1/8" punch, tap-out the pin that penetrates the planetary gear cover and the motor shaft (takes some finesse to avoid damage to the painted/coated planetary. It is a tight fit!) This will allow the planetary gear and cover to drop.
Now the motor and gear unit can be removed. Do so by removing the Phillips-head screws securing the motor/gear unit.
Remove the Phillips-head screws securing the speed-control circuit board. Free it up to be able to access the bottom of the board (to unplug wiring).
Now, disconnect the wiring that connects the speed control circuit board from the motor and remove the defective board/speed control.
Connect wiring for the NEW circuit board to the motor unit, and re-attach both the board and the motor into place using the appropriate screws.
Insert the planetary gear unit back on the motor shaft. Line up the holes for the pin and carefully tap the pin back into place.
Replace the top of the motor enclosure and the decorative trim piece.