Is it the screen, or the screen protector ? If the latter, just remove it and replace it. If it is the actual screen, first of all try a lint-free cloth barely dampened with distilled water, and test it on a corner, see if that works. On no account get the camera wet. If that is unsuccessful, try adding a very little diluted washing-up liquid, and try again. Should that not succeed, I would advise taking the camera to your local repair facility for their expert advice.