Hi John,
I'm not entirely sure what you're asking.
If you want to get rid of the demo install, you simply need to drag the X-Plane folder to the trash. You can empty the trash to completely delete it and regain the space on your hard drive.
Or are you saying that your desktop background is a picture of a cockpit? In that case, you would need to go into your system preferences > desktop & screensaver & change it there.