That sentence comes from Harry Potter 1, and while most kanji have furigana, 上下 doesn't, meaning that the reading should be obvious even to a kid. Or maybe it means that any reading is OK... I'm a bit confused!
[maybe I should worry about my English first, since I even spelled wrong the title of the thread...^^]
上下 has two common readings.... うえした and じょうげ. It seems that うえした is used only as a noun. じょうげ is used as a noun and as a suru verb. So if 上下 is being used as a noun, it would seem that either reading is correct.
I doubt that either reading is correct. I'd imagine that one will be appropriate given the context. Consulting Kenkyusha's online dictionary, the entry for じょうげ is at least 10 times as long as any of the other pronunciations. So the chances are that じょうげ is the correct reading.
うえした seems to carry the meaning of upside down quite often.