Go to your thermostat and turn the switch to the "Off" position. Listen for the fan to turn off before you locate your air handler.
Locate the service door on the air handler. This should have two rings that serve as latches for the door. Turn the two rings counterclockwise to unlock the service door.
Pull the door open to access the air filter. Pull the air filter from the slot and make a note of the way that it is sitting. The arrow on the filter will direct you in how to install your new filter.
Install your new filter according to the arrow on the filter. Replace the door and the latches to complete the installation.