Can someone tell me how to clean a Silver Certificate?

You can not improve the value of the note by cleaning it but if you just want to make it look a little better you can soak it in warm water with some dish detergent. Let it soak overnight and then soak it briefly in some clean water to rinse it. Place it between 2 paper towels and let it dry. FYI - the bill was printed after 1935 but before 1957. The D indicates the 4th change of either Secretary of the Treasury or the Treasurer of the United States.