It's a relatively simple procedure, if you have a pair of fine-point tweezers:
From the right in the photo, unscrew the red plastic collar with your tweezers (lefty-loosey). Lift out the spring, and the white switch. Use your tweezers again, to unscrew the brass collar (lefty-loosey). This is fairly tight, and should be lubed when re-installing. Pop out the switch boot.
Reverse the process to re-install.