It depends what is on it. If the reflective surface is in otherwise good condition, you can try a little bit of water with a small amount of dishwashing fluid. Follow up by cleaning the surface with some optical cleaner.
There is undue and unnecessary concern with scratching mirrors. As long as the reflective coating hasn't been damaged, you don't have to worry too much about cleaning. If the coating is in poor condition and flakes off when touched with a wet swab (try this on a corner), then that's something very different, and eventually you will want to replace that mirror.
Remember, the mirror has no role in forming the final image. It simply reflects the scene on to the viewing screen. That's all. Minor scratches (or even a deep scratch) will have little -- if any -- discernible effect on the viewing image.