The answer to 'how to fix furnace' is to first check the air filter and replace it if it is dirty. Then, inspect the thermostat to ensure it is set correctly and the batteries are working. Next, check the pilot light or igniter to make sure it is lit. If the furnace still isn't working, you may need to clean the burners or check for any blockages in the vents or ducts.
It's important to follow all safety precautions when servicing your furnace, such as turning off the power and gas supply before performing any work. Regular maintenance, such as cleaning the furnace and replacing the air filter every 1-3 months, can help prevent issues and extend the life of your furnace.