Do not yank the upper case off quickly. The case is attached to the logic board via a ribbon cable.

Lift up at the rear of the case and work your fingers along the sides, freeing the case as you go. Once you have freed the sides, you may need to rock the case up and down to free the front of the upper case. This stage can be quite tricky. Over the DVD reader are 4 tabs set back which pull out vertically.

Note that the two small tongues on the left hand front of the upper case may bend while you remove the upper case. When re-installing, you may need to bend them back to fit in the grooves in the lower case.