Summarize:

Make sure that the pan/wok is big enough to fit all of your stir-fry. If it is too small, the food won’t reheat properly and turn out soggy. For best results, use a flavorless cooking oil, such as vegetable or canola. Avoid coconut or olive oil, as they may affect the final flavor too much. If you have different types of stir-fry to reheat, stick with just one type for now. You don’t want to mix the different flavors.  Spread the food as evenly as you can across the bottom of the pan. Watch for oil splatters. The pan may sizzle when you add the stir-fry to it. This might seem a little unusual, but it’s the key to reviving the flavors in your meal. It will also help rehydrate the sauces that may have dried out during storage. Without the water, the stir-fry will turn out too dry.  The larger your portion is, the more water you will need to use. Have a measuring cup filled with water nearby. You will be adding water gradually to the stir-fry as it cooks to keep it moist. As the food cooks, the water will begin to evaporate. If the food is not cooked through, then you will need to add another 1 to 3 tablespoons (15 to 45 mL) of water to rehydrate it, and cook it longer. Just keep cooking, stirring, and adding water until the food is hot. This should only take a few minutes.  Stir the food often with a wooden spatula. If you don’t have one, use a large, wooden spoon instead. How often you add the water depends on how dry the food looks. If the food starts to look dry and stick to the pan, it's time for more water. How long this takes will depend on how cold and dry the food was to begin with. The colder and drier the stir-fry was, the longer this process will take.
Heat 1 teaspoon of oil in a large pan or wok. Add the stir-fry into the pan or wok. Add about 1 to 3 tablespoons (15 to 45 mL) of water into the pan. Cook and stir the food, adding water as needed, until it is hot.