How hard is it to change a fuse box?

If you have to ask that question you are not qualified to do the job. First it is about doing it safely. Then it is about knowing the color codes of the wires. Also it concerns getting the right new box and exchanging it for the old box. You could go to a diy page and get the details. This answerer has done this many times and does not recommend it to an armature without qualified, licenced, supervision.