Place the CygoLite on a flat surface with the bottom side facing up.
Locate the four screws on the bottom of the device.
Unscrew them with a Phillips head screwdriver and remove the bottom casing.
Locate the rubber gasket on the side of the device.
Place your new replacement rubber gasket onto the side of the casing and gently place the top of the case back on top of it.
Be sure to line up the grooves in the casing with the grooves on the rubber gasket.