You just unscrew the retaining ring with a ballpoint pen or needlenose pliers, then pull out the switch guts, then there's your switch boot, ripe for the replacin'.
Usually the tailcap has a retaining ring, either metal or plastic, with one or two holes in it. Place something in those holes, overall, pliers seem to work best, and unscrew the retaining ring. Then push the parts out(push from the outside) and do what you need. Then go backwards to install everything back again.