3-a. (Replacing main board):
By using the nipper pliers (put some fabric things on two ends to avoid any damage to the product), clip
the front pushbutton and pull out by turning upwards, then to the right to take out the front damper by
tweezers or just fingernail, see picture below:
3-b. (Replacing main board):
Take out the knob, and use cross-head screwdrivers to take out the two screws, as shown below:
3-c. (Replacing main board):
Pull out the upper cover, and take out the pushbutton, see below pictures: