Removing the horizontal scroll wheel assembly is tricky and may take a few tries—be patient and work slowly.

Firmly secure the top shell with one hand.

Use your free hand to rotate the horizontal scroll wheel assembly away from the shell and lift to remove it.

During reassembly:

Make sure the cables in the center of the shell are out of the way so they don't get trapped under the horizontal scroll wheel assembly.

Put the assembly back into place so the horizontal scroll wheel and side buttons go into their slots and the plastic housing's circular cutouts go over their pegs.