In addition to the screws which hold the cabinet to the floor. If you take off the front cover you should see 2 screws which hold the furnace works to tracks inside the cabuinet. Take out these screws and the external inlet / outlet cover, gas line , thermostat and 12v
lines and the inner works slide out. If the furnace is a NT22 there was a safety recall. Many parts will not be available and it is next to imossible to get anyone to work on these old furnace.. If you are putting in a newwer furnace you might want to save the lower cabinet for ir its ductwork connection.