I live in a house that used to belong to a DIY'er. Problem is, there isn't
anything he did right. I am in the middle of fixing his mess in the garage.
HEre is the situation: The main breaker is in the basement .it is a 20 Amp
dual breaker. THere is a subpanel in an upstairs closet (don't ask me why)
with 2 20 Amp fuses. One of the fuses runs to the garage. The garage is
mixed 14guage and 12 guage wiring. The other fuse runs to the family room
where hot and neutral appear to be reversed.

For now I'm doing the garage. First thing: install GFI, Second thing:
simplify the rats nest of wiring (the box going to the door opener has 8
pieces of 12 guage romex going to it),

My question: to resolve the mixed guage issue: Can I just replace the fuse
in the sub panel with a 15 amp fuse? and leave the 20 Amp breaker in the
panel. All that is in the garage is the opener, 2 outlets and 2 lightbulbs.

why would there be a ganged breaker for this circuit? I can only think of a
shared neutral unless one leg goes to the garage and the other goes to the
family room or the guy was a complete doorknob and just put it in. I
probably will get into this when I see why hot and neutral are reversed.