This is what you are going to find inside. Next step is release primary and secondary buttons (piece nº 2) pulling up straightly and softly, being carefull do not break a and b parts that hold the button.

Other piece we must take care is piece number 5, which is quite fragile because is glued to the board through 2 thin wires that could brake. This part keeps wet around the wire and we must dry it perfectly keeping it warm.

Point number 6 shows how board engages the pen case. This is why we might no turn too much pieces 3 and 4 between them in step 1 to keep us avoid to break the circuit board.