If an air plant has brown spots it's because it hasn't received enough water. The good news is that the plant isn't dead. The first thing you can do is soak the plant in water.
If you use tap water, make sure you dechlorinate it before soaking the sick air plant in it. The best water to use when curing a sick air plant is rainwater.
Another problem is that the air plant needs more air. If you have it in a container with a small top, find something that has a large opening that will provide the plant with more air.