Replacing this button requires soldering. I recommend a "needle-type" soldering iron head to push the pins of the broken button out of the PCB.

See this guide for the initial teardown. I will pick up where Brett leaves off.

You will need to purchase a replacement button (Digi-Key -- EG4414-ND). But before purchasing, tear the mouse down and confirm the button is broken and it is not another problem.

Required tools:

multimeter (to confirm button malfunction)

soldering iron, solder, flux, wick, isopropyl

required screw driver heads