1. Remove the plastic cover right below the screen.
2. Remove the keyboard.
3. Locate the screw holding the optical drive in place and remove it.
4. Pull out the drive and insert your brand new one.
If you need a more detailed...
Originally Posted by mr_Ray
I've taken out every single screw I can find on the bottom of the case, but I can't seperate the top from the bottom - it seems to be joined at some point in the centre.