The aspect ratio of a high-definition picture displayed on the television is determined by the signal being received. The television will automatically adjust the aspect ratio when viewing a high definition broadcast. The aspect ratio can only be changed on the television if the signal being received is 480p or 480i. Otherwise, it cannot be changed on the television.
Some televisions have a Zoom or Wide option which will allow you to fill the screen with the image. Refer to your owner's manual for more information.
NOTE: If using a digital set-top box, the aspect ratio can be changed from within the menu of the set-top box. Consult the instruction manual for the digital set-top box for further information.