As far as I'm aware it'll get sent away even if you take it to an Apple store. I don't think there are any on-the-spot repairs or replacements. Your best bet, if you have a store nearby, is go there. Otherwise, contact Apple via phone or the online support system to arrange repair and they'll have you post it to them (in Australia, they do this by sending you a shipping label via email that you then take to the post office with the watch and they do the rest - not sure how it works elsewhere).