It sort of depends on which fogs you have- the short plastic ones or the longer ellipsoid ones.
The ellipsoids are pretty easy to take apart- there are 4 screws that hold the lens assembly to the body- remove these and the 2 parts will separate easily. Reinstall the gasket carefully to prevent future leaks.
The smaller plastic fogs are pretty difficult to take apart without breaking something. Maybe some heat (hairdryer) would help- but I have not tried that.
Bret.