Procedure:The air filter case is a two-part shell with the air filter element between the two halves. The two halves are held together by one screw at each corner (four corners, four screws). The screws are threaded through the upper shell, into metal fasteners on the lower shell.
IN SOME CASES, THE METAL FASTENER CLOSEST TO THE POSITIVE BATTERY TERMINAL WILL BECOME CORRODED BY GASES RELEASED BY THE BATTERY.
If this has happened to you, you will know, because as you try to unscrew the screw closest to the positive battery terminal, the metal fastener will rotate with the screw, and will not allow you to loosen the screw or open the airbox.
To fix this, I highly recommend using a spray-on lubricant on this fastener and letting it sit for 5-10 minutes before attempting to replace your air filter. Personally, I spray all four fasteners with PB B'Laster before servicing my air filter.
If the fastener still turns with the screw, then you need to grip the lower fastener with a pair of pliers or vise grips.