Note that this section is intended for replacement of a standard through shell, low voltage threaded plastic spa light lens.  This is the most popular spa light lens in use for the last 10 to 15 years.  Other types of spa lights may be similar, but the installation may not be identical.  If you are unsure as to the suitability for use of these instructions for your application, then do not use them!
 

Usually what happens with these things is that they will crack after a long period of time, particularly if exposed to excessive temperature differentials, like, very cold water with a hot bulb, or if the sealant or the hole had created a situation where the plastic lens tried to expand, but couldn't. You'll want to start by draining the tub first, and turning off all power to the tub.  Then, carefully pry the cracked lens pieces out of the spa shell.
This is all pretty much self explanatory.  When one of these things goes... it really goes.  So remove all left over remnants of the old lens fixture.  If you have a full foam spa, take extra care to not crack the shell at the area of the light hole.
If there are traces of sealant, remove them from the surface by rubbing them with your fingers.  It is also a good practice to clean these surfaces with rubbing alcohol to remove all contaminants from the surface of the shell.
These are the components of a new light lens assembly.  The lens, a gasket, and a lock ring.	I personally prefer to use a small amount of silicone sealant here, even though the gasket alone usually works fine without it.  Warning! DO NOT USE an "Acrylic" or siliconized acrylic type of sealant!  Use ONLY 100% silicone
Lens with silicone applied.	Lens with silicone and gasket, ready for insertion into the lens opening in the spa shell.  If there's a rib on the gasket, you'll want the rib facing the shell, not the light.
Push the lens into the spa shell.  You may have to "thread" it in place by turning clockwise.	On this particular spa, the lens back is accessible through the light access hole, seen here behind the pump motor.  Using the black lock ring, thread it on by hand only.  Tighten down the lock ring enough to get the lens firmly seated in the spa shell.  Don't use a wrench!  You could cause a problem with the lens, the shell, or even force the gasket to expand past the outside perimeter of the lens.
	With the lens seated properly, use a rag to clean up the excess silicone sealant around the light fixture.  Do this quickly before the silicone has a chance to cure on the surface.  Once you've achieved the desired coverage with the silicone sealant, leave it alone until it finishes curing.  Don't use a knife to remove excess sealant.  It's better to remove it before it cures, than to risk losing the integrity of the seal after it cures.
Be sure that you give the sealant adequate time to cure completely, preferably 24 hours.  Refill the tub, check for leaks, connect the light, and enjoy!