Starting near the display and working around to the front of the computer, pry up on the upper case. It is held with clips on the right above the optical drive. These will release with some firm lifting pressure.

Be careful when prying up the upper case. It's very easy to slice open a fingertip and thus provide the blood sacrifice the Mac gods sometimes require of those who insist on doing their own repairs.

There's a trackpad and keyboard ribbon connecting the upper case to the logic board, so don't pull the upper case off entirely just yet.

If you have trouble getting the clips to release, be careful that you are not prying the plastic top of the upper case away from its metal frame.