iFixit

Remove the two 4.5 mm Phillips screws on either sides of the battery contacts.

Breathe deeply. Trying times are ahead, but we promise the lower case does come off.

Push the thin rims of the lower case surrounding the battery compartment in, bending them past the tabs, and then lift up to free that corner of the lower case.

There is a slot on the wall of the battery compartment that locks the lower case in place. Use a small flathead screwdriver to pry out the slot's lower rim and pull up on the lower case to free the slot from the tabs holding it.

Run a spudger along the seam between the lower case and upper case on the front of the computer to free the tabs locking the lower case. Pull up on the lower case and continue to use the spudger as necessary until you hear three distinct clicks.

Continue to run the spudger around the front right corner. There are two tabs on the port side of the computer, one near the front corner and one near the sound-out port.

There are three tabs over the optical drive that must be released before the lower case can come off. Slide the spudger into the lower case above the optical drive and run it toward the back of the computer until you hear three distinct clicks.

The front and sides of the lower case are now free.

Turn the computer so that the back is facing you and pull the lower case up and toward you until the back tabs pop free.

It may be helpful to jiggle the case up and down.

Remove the small greasy springs with white plastic caps from either side of the battery contacts.