how do I unplug fuse box 12v cable ?

If you are trying to make the fuse box safe to work on, disconnect the battery negative (ground) wire in the back of the car where it is connected to the bodywork.

 

I assume that you are referring to the main relay/fuse box next to the inverter in the engine compartment, and you want to disconnect the +12V cable which connects to the dedicated positive jump start terminal and is covered by the hinged plastic red cover.

If my assumption is correct, then you need to use a 10 mm socket to remove the nut that secures the cable. After the nut is removed, use a pair of long-nose pliers to pull the cable up and away from the stud.

 

If your Prius does not come with front fog lights, then the fuse box will not have the fuse.