to release the trackpad top case, carefully but firmly push on the under part of the of the top case. (This will free the internal latches you can see on the inside of the battery bay).

repeat to push on the underside for the middle and left side of the case. The left side is a little harder to push. On the top there also is a little latch that catches onto the hard disk cage underneath. Then lift the top from the case.

Be aware that by now all 90s plastics have become very brittle, so take care.