They can be fixed easily with our contact cleaning brush with no great technical knowledge. Most camera SLR mirrors are easily removed with tweezers and replaced without taking the camera apart. Camera makers conceal the screws used to take them apart under leather. Peel it back to expose the screws. Sometimes glued-on shiny plates conceal the screws.
Cameras often use a ring without slots.
It is removed with a clamp wrench or auto hose clamp covered with tape . Never oil a camera heavily. Camera oil is very thin special watch oil. We sell it. Parts for cameras are not hard to find.