Slide the vent panel down the tracks, near the center of the door opening. Locate the screw cover on the vent panel's edge. Pop the cover off with a small flathead screwdriver. Insert the screwdriver into the hole and turn it counter-clockwise, as far as it will go, to lift the rollers and lower the panel.
Locate the screws securing the Pella sliding door's head stop in place. The head stop is the piece that sits in front of both panels, helping hold them in the tracks. Back out the screws and set the head stop aside.
Lift the panel up so the bottom edge clears the top of the bottom track. Pull the panel's bottom edge toward you. Once the panel clears the bottom track, lower the panel so it comes out of the top track. Set the panel aside.
Cut along the seam where the fixed panel meets the frame with a utility knife. This cuts through any paint which may be holding the door in place. Look around the fixed panel's perimeter to locate all the screws.
Back out the screws holding the fixed panel in place. Slide the panel toward the center of the door opening. Lift up on the panel and remove it the same way you removed the vent panel.