Available from Nissan: Part# B6553-89915 "sealer.
Excellent sealant. Remains flexible so it doesn't get hard and lose its seal. I suggest using this when you reseal the headlights, much better than typical silicone or RTV silicone.
In order to take the lights apart, you need to remove them from the vehicle. Remove all of the bulbs and loose pieces, and drain as much water as you can out of the light.
Next, you can either use your oven (Google "baking your headlights"), or a heatgun to heat up the seal around the edge of the light, and pull the plastic lense off. Remove the sealant in the groove and on the lense, apply new sealant and put it back together.