The answer to 'how to fix air handler' is to first identify and troubleshoot the issue. Common air handler problems include clogged air filters, failed fan motors, and issues with the control board. Start by checking the air filter and replacing it if it's dirty. Next, inspect the fan motor and ensure it's running properly. If the motor is faulty, you'll need to replace it. Finally, check the control board for any error codes or signs of damage.
If you're not comfortable troubleshooting the air handler yourself, it's best to call a professional HVAC technician. They have the expertise and tools to diagnose and repair the issue safely and effectively.