The circuit board is a small part inside the back of the mixer. It is fairly easy to get to and doesn't require many steps.
Using a Phillips #2 screwdriver, remove the 10mm screw on the back cover by rotating it counterclockwise.
The back cover is now free. Remove it by pulling it back and up.
Using a Phillips #1 screwdriver, remove the 9.5mm screw above the circuit board by rotating it counterclockwise.
Using pliers or tweezers, remove the connectors of the four wires attached to the circuit board.
Grab the wires and tilt the edge of the board up to lift the circuit board up and out.