The answer to 'how to fix air filter' is to replace your HVAC system's air filter regularly. Air filters should be changed every 1-3 months, depending on usage and filter type. A dirty, clogged air filter can reduce airflow, increase energy use, and lead to more frequent breakdowns.
To replace the air filter, first locate the filter, usually in the return air duct or the air handler unit. Turn off the HVAC system, remove the old filter, and insert a new filter of the same size and type. Make sure the filter is installed with the airflow arrow pointing in the correct direction. Once the new filter is in place, turn the system back on and check that the new filter is seated properly.