The fuse is located at the bottom of the unit for DC20. You need to lay the unit on the side and use a flat head screwdriver to unscrew the fuse case and replace the fuse if needed.
For C100+, C150+ & C20, unplug the cord from the unit and you can use a flat-head screwdriver to open the fuse case. Remove the fuse carefully to prevent damaging the fuse case.
Click this link for video instructions for DC20. If you need a fuse replacement, you can buy it at any hardware store near you and show the old one you have.