The answer to 'how to fix heat exchanger' is to have a professional HVAC technician inspect and repair the heat exchanger. The heat exchanger is a critical component of a furnace that transfers heat from the combustion process to the air circulated through the home. If the heat exchanger cracks or becomes damaged, it can allow dangerous combustion gases to leak into the home, posing a serious health and safety risk.
Attempting to repair a heat exchanger yourself is not recommended, as it requires specialized skills and equipment. Trying to do this work without the proper training can be extremely dangerous. Instead, you should contact a licensed and insured HVAC contractor to diagnose and fix any issues with the heat exchanger.