The button is attached to the 3d printed case using a bracket and two M2 x 5mm flat head metric machine screws. Press the button through the hole in the side of the case. Hold it in place and fit the bracket over the body of the button.
Insert and fasten the machine screws through the mounting holes on the side of the 3d printed case. Make sure the mounting holes on the bracket are lined up with the holes on the case.