The answer to 'how to fix heat exchanger' is to replace the heat exchanger if it is cracked, corroded, or damaged. A malfunctioning heat exchanger can be a serious safety issue, as it may allow dangerous combustion gases like carbon monoxide to leak into your home. Attempting to repair a damaged heat exchanger yourself is not recommended, as it requires specialized tools and expertise.
If your HVAC system is exhibiting signs of a faulty heat exchanger, such as strange noises, unusual smells, or reduced heating output, it's best to call in a professional HVAC technician to diagnose and replace the heat exchanger as needed. They can safely remove and dispose of the old unit, and install a new heat exchanger that meets all safety and efficiency standards.