How do you heal a brown cactus?

More water or less sunlight may be ways to heal a brown cactus plant.

Specifically, a cactus is drought and heat tolerant. But it may turn brown because of too little water. The plant is adapted to water-poor environments. But that does not mean that it has no water needs at all.

A cactus plant likewise may turn brown because of too much sunlight. The latter situation is the equivalent of a cactus sunburn. A cactus in fact has a side that adapts to receiving more sunlight than the other. But there is such a thing as overexposure if the sunning lasts too many hours and the light is too harsh.