As long as it's not a pellicle mirror, just clean it with a clean dry white tissue (no scents, no lotion) and some eyeglass cleaner.
First use some compressed air to blow away debris. Spray away from the camera once or twice before blowing the air on the mirror.
Spray the cleaner onto the tissue and wipe off the mirror.
Wipe, turn the tissue, wipe again. I use some long tweezers or forceps for this job.
If you have black gunk stuck to the mirror, that's the foam bumper, which should be replaced. It's also an indication that the foam seals should be replaced. If that is the case here. If you do have black gunk on the mirror, you probably will have to use some lighter fluid to remove it. Then clean the mirror as described above.
The mirror has no role in forming the image on film, so just clean it with the usual amount of care.