I think you can try to remove some surface damage with a wooden toothpick (or a porcupine quill as some people here suggested). When done with that, you can try with a lukewarm bath with some mild soap and see what happens. I got recommended a soft toothbrush or cotton wool. If this doesn't happen, I think most here would suggest to soak it in acetone for a while. Pure acetone, that is, so no mixture with 90% acetone and 10% rubbish or so.
EDIT: Actually, I think most people here would recommend not to clean the coin. Given that this one here is not very special or very valuable and already in bad condition, it may also be a great way to just get experience a bit with the whole cleaning thing.
Edited by UltraRant
02/06/2017 6:42 pm