You can unbolt the stock airbox, and fit a cone filter to the bottom of the MAF tube. If you go to Autozone or other parts suppliers there is a company that makes a K&N look-a-like cone filter that will fit. Here's a How-TO. I did this to mine, it's worth a few hp and 2 mpg or so depending on how you drive.
As was stated above, the filter element is NOT REPLACEABLE, the unit is sealed.
In the response above, click on the "HOW TO" link and replace the complete stock unit with a cone type filter as described. I did mine and love it. [thumb] [thumb]
The instructions are straight forward, and will take you a couple hours.