Are you talking about replacing the hard drive on a MacBook? Whether or not you can do it and the procedure for doing so will depend on which model MacBook you have. Go to this page and put your serial number in the search field, and it will tell you what model you have. Then look at the manual. The ones with replaceable hard drives have entries in the manual telling you how to do it.
Best of luck.
Here are instructions on replacing the hard drive in a MacBook with a removable battery. http://creativemac.digitalmedianet.com/articles/viewarticle.jsp?id=45088
Here are video instructions on replacing the hard drive on the Aluminum Unibody
Here are video instructions on replacing the hard drive on the White Unibody http://eshop.macsales.com/installvideos/macbook_13_09_unibody_hd/