Hi Paul, replacing the light niche is a very involved process. You would need to have access to the rear of the niche to glue the conduit in and this is only possible during initial installation or if you are doing an extensive remodeling of the pool.
If there is a small leak at the conduit fitting you can use a Cord Stopper to plug it up. If the leak is around the face of the niche, this AquaMend Underwater Repair Epoxy Putty should seal it up if you have a gunite pool.
Any replacement light fixture will be specific to the niche you have. Let us know what it is if you would like us to recommend a replacement.