Turn off your heater if you are installing the filter on an heater. If you are installing the filter on an air conditioner, turn off your air conditioner.
Reveal the old air filter by opening your heater or air conditioner's access door. On a heater, it is usually located in the front, near the bottom of the unit. If you are changing an air-conditioner filter, there is usually a metal strip that you can slide out of place to access the filter. On some units, this covering will be secured with screws and it will be necessary to use a screwdriver to remove them.
Remove the old filter from the heater or air conditioner while taking special care to note the arrows on the inside of the unit. These arrows indicate the direction air will be pushed through your filter.
Slide your new filter into place according to the direction indicated by the arrows.
Lock the cover you removed to access your old filter and put the old filter in the garbage.