It depends on what you mean by repair. If the guitar is an expensive or rare model, take it to a professional. Chances that an amateur will damage or diminish the value of the guitar are better than good. If the finish is just worn from use, leave it be. No one will fault you for having a well worn guitar. Take a look at some of the great guitar player's instruments, and you'll see that they have no qualms about the wear on their guitars. If its an inexpensive guitar and you aren't particularly concerned about damaging it, google it. There are complete re-finishing kits and tutorials readily available.