I'm not an "expert" but I think that there isn't any way to really restore the silvering. Even if there was a way to restore the bad part there would still be optical differences between the two areas and at their transition that would look pretty weird (crappy). It is usually a pretty straightforward proposition to completely replace most mirrors. There is some cost related to having one cut and purchased from a glass company. They could also install it for you. If you need advise on how to remove your old one and how to install a new one it would depend on how it is attached. Is it glued to the wall, held in by clips or integrated right into a door or vanity? I would assume that you need to replace the mirror.