Swing the bottom of the touchpad up and away from the controller to remove it.

During reassembly:

Insert the touchpad through the back of the case and hook the top edge into place.

Make sure the touchpad cable doesn't get trapped between the case and the touchpad.

Push the bottom edge of the touchpad firmly into place, making sure the circular alignment posts snap into place and are sitting flush in their cutouts.

Check the front edge of the touchpad and make sure the thin strip of plastic on the front shell isn't wedged under the touchpad. If it is, remove the touchpad and reinsert it so the thin strip is parallel to the touchpad.