This is a sticky one. If you turn it off to end the update, even though it says 100%, the way you would shut it down would be like pulling the battery on it which is not good for your device or its data. On the other hand it could be sitting there in an endless loop... I say error on the side of caution and wait it out. Once you feel you can wait no more or it's been long enough you could try forcing it off, but that should be your very last resort.