It usually depends on the context. In titles or when referring to a specific genre or style, 'Science Fiction' might be capitalized.
Generally, 'science fiction' is not capitalized unless it starts a sentence or is part of a title. For example, 'Science Fiction is my favorite genre.' But in normal text within a sentence, it's 'science fiction'.
The capitalization of 'science fiction' varies. If it's a proper title of a book or a movie, like 'Star Wars: A New Hope in the World of Science Fiction', it's capitalized. Otherwise, in regular writing, it's not. So, it all comes down to how it's being used in the specific text.