To maintain a refrigerator properly, you should keep it at a temperature of 40°F or lower. This temperature range helps to preserve food by slowing down the growth of bacteria that can cause spoilage or foodborne illnesses. It is important to regularly check the temperature of your refrigerator with a thermometer to ensure it stays within the recommended range. By maintaining the refrigerator at the correct temperature, you can keep your food fresh and safe to eat for a longer period.