Mix up some baking soda and water. Apply it to the terminals and it will fizz up and dissolve the acid. Then clean rhe battery post's and cable ends real well with a wire brush ( or sandpaper will work too). Apply a thin coat of Vaseline and reassemble. They make special brushes for cleaning batteries and they make various products to spray on everything to keep the corrosion at bay. You can find them at any auto parts store.