To change the air filter, first, you need to locate your filter cabinet, which you will find next to your furnace or air handler. Next, you need to unwrap your Lennox air filter and pull the handle of the cabinet's front panel to remove it.  Then, you need to slide the filter out using the cardboard handles. The last step is to insert a new filter while making sure that the airflow arrows are pointed toward your furnace or air handler. Lastly, you need to ensure its replacement every 9-12 months.