You can defrost them in the refrigerator overnight, and then refresh them by softening them in a hot skillet for a few moments on each side. Be sure the skillet is very hot before you place the tortilla on it, or the moisture on it will cause sticking. You can also cover the frozen tortillas in a wet paper towel and pop them in the microwave until they're just pliable. It usually takes about 30 to 45 seconds, depending on your microwave's wattage, the size of each tortilla and how many tortillas are in the stack.