As it turns out the exact reheating procedure for Krystal burgers is not listed online. You may try the procedure below - 'how to reheat hamburgers' - from Livestrong.com:
Prepare the hamburgers for reheating by letting them get to room temperature. You should not leave cooked hamburger at room temperature for more than two hours, explains the Food Safety and Inspection Service. This will help prevent food-borne illness.
Wait until sandwiches get to room temperature before trying to separate the buns, condiments and meat if you are reheating whole sandwiches, rather than just patties. Cold buns may stick to patties and tear when you try to remove them.
Remove buns and any condiments when the sandwiches are at room temperature.
• Set your microwave to the 'reheat' setting. This will warm the patties from the outside, rather than cooking them from the inside.
• Place the patties on a microwave-safe plate.
• Place in the oven for one to two minutes, depending on their thickness, then flip.
• Press a patty gently with your finger to see if it is hot on the bottom. If it is, it is finished reheating. If not, reheat patties for another 30 to 60 seconds, depending on how warm they were after the first minute. Sliders will take much less time, usually a minute or less.
• Remove the patties from the microwave and place the buns in to heat for 20 to 30 seconds. Bread will quickly overheat, especially on a high setting, becoming hard and crusty within minutes of removal from the microwave.
Please note that if you are using a high setting, do not put the buns in for one minute and walk away; within 15 seconds, they should be done.
You might also want to check out this White Castle Slider reheating instruction (restaurant-bought Sliders):
1. First, don’t heat the pickles. They get very hot and could burn you.
2. Remove sandwiches from cartons, place on a microwave-safe plate and cover with a paper towel.
3. Heat on High for 15 – 20 seconds per sandwich, if refrigerated. 30 seconds per sandwich, if frozen.
*For best quality, they recommend refrigerating vs. freezing the restaurant-bought Sliders, as freezing can cause the buns to get very dry and hard.
If you want your leftover fries to be crispy like they were when you bought them, a microwave isn’t the appliance to use. As per Leaf.tv, microwaving fries makes them warm, but limp and soggy. Here's how to reheat fast food french fries - quick and crispy from eHow.com:
1. Remove the french fries from the fridge. Place on a microwave-safe plate that is covered with a layer of paper towel. Lay the fries on the plate in a single layer. Heat them in the microwave for 20 seconds.
2. Pour 1 tbsp. of oil into a frying pan. Turn heat on high. <br
3. Place fries in pan. Cook each side for approximately two minutes and then flip.
4. Drain the oil from the fries by placing them on a paper towel.
Leftovers can be kept for three to four days in the refrigerator. Be sure to eat them within that time. After that, the risk of food poisoning increases. If you don't think you'll be able to eat leftovers within four days, freeze them immediately.
Know the basics of leftover safety at Foodsafetynews.com.