So basically, officially, and easily, you cannot, however if you think you're at the right skill-set, and have the time/patience, then use the [http://www.ifixit.com/Teardown/MacBook-Pro-15-Inch-Unibody-Late-2008-and-Early-2009-Teardown/590/1|teardown] and try to do it yourself. You'll probably also be voiding warranty, so unless you're rather experienced and not under applecare I wouldn't suggest you do it.