Please tell me if you can reverse a small cavity?

Not really: Microscopic areas of demineralization can be remineralized. But by the time a cavity gets to be where it can be seen with the naked eye or on an x-ray the infected tooth material has to be removed and a filling placed to replace the damaged tooth structure. The sooner a cavity is fixed, the smaller the procedure, the smaller the filling. If you think you have a cavity please call your dentist.