Consider before you do -- the enclosure may not have enough power to handle a larger drive, depending on the age and the model.
It's just a press fit. You can pry the bottom off using something thin (like a kitcken knife).I've got a la Cie Porsche hard drive and the hard drive is finished. I've toyed with the idea of replacing the hard drive. Any ideas on how to open the enclosure without damaging it. (There are no visible screws to unscrew.)