Located in each airline's main office, on the corner of the desk is a model plane - click on it to see the screen where you can retrofit your planes with one to three-star technology (three stars being the best but most expensive option). By default, new planes only come with one-star technology.
Along of the top of the screen there is the name of the aircraft you would like to retrofit. Just below that are a set of controls. The "OK" button will exit the retrofitting screen. Immediately to the left and right of the "OK" button there is a letter "M" with a down arrow and another letter "M" with an up arrow, respectively. The former saves the current configuration, and the latter loads a saved configuration (more on this later). Further to the left and right of the "OK" button there is a button with two triangles pointing to the left and another with two triangles pointing to the right, respectively. The latter cycles forward through the planes in your fleet; the former cycles backward. Beneath these buttons there are two single triangles, one pointing to the left and one pointing to the right. Clicking on the left one shows the more "exterior" aspects of the plane while the clicking on the right one shows the more "interior" aspects of the plane to configure. This means that if you would like to repeat a particular configuration, you can cycle to an aircraft with the configuraion you want replicated, click on the "M" with the down arrow/triangle, then cycle to another aircraft, and click on the "M" with the up arrow/triangle to "load" the configuration onto the other aircraft with less hassle. This excludes "class" configurations and number of flight attendants assigned to the plane.
Beginning on the left side of the screen you can manage more of the "exterior" aspects of the plane including (from top to bottom): Landing Gear, Lights (although the game refers to "Engines"), Safety/Flotation Devices, and the Flight Computer System. At the very bottom, you can adjust the proportion of first and second class seats available on that aircraft.
Clicking on the single triangle pointing to the right will take you to the right side of the retrofitting screen where you can configure the "interior" aspects of the plane. This includes (from top to bottom): Seat Comfort, Luxury Articles, Ambience, and Food/Meals. At the bottom, you can adjust the number of flight attendants serving aboard this aircraft. The number of greyed-out flight attendants indicates the number of free spots available aboard the plane for flight attendants to serve. The number of flight attendants with red hair indicates the minimum number of flight attendants required for the plane to fly. Without enough flight attendants, the plane will be grounded and any flights in its schedule will be cancelled. The number of flight attendants with yellow hair indicates the number of optional flight attendants on board.
As mentioned, when you're done retrofitting the plane, click "OK" - the plane will be refurnished and you will be charged the fee for it.