The DC20 PCB is located inside the unit under the handlebar so you'll need to remove everything to reach and replace it.
For all other models, open the unit and locate the PCB near the cord socket at the rear end.
You'll have to remove each part (check valve, motor, battery, & tank) and make sure to take a video/photo to put everything back in the right place.
You can click this link for a video instruction for DC20 and this link for all other models of ULV fogger.