How to store
You can keep a ripe pitahaya for a couple of days in the fridge, but the best is to eat them quick as possible. An unripe pitahaya could get ready while keeping it at room temperature.
How to eat
Scoop the pulp from the skin with a spoon. Then cut the decorative pulp into cubes or slices. Or use a melon scoop to make nice pitahaya balls of it. You can of course also eat the fruit straight from the skin, like a kiwi.