What do you want to change?
- The original material as recorded by your camera?
- Or a 4:3 project you are already working on in PE?
When you start a new PE project, you should specify the format: 4:3 or 16:9 (widescreen).
You cannot change these project settings inbetween. As a workaround: save your video on the timeline as an .avi; start a new project with the required different settings and import this .avi in the new project.
You can input 4:3 footage in 16:9 projects and vice versa, but PE will then crop the image or add black lines to it.
Also you can share (export) a 4:3 project as a 16:9 movie and vice versa, but the same will happen.
Easiest to start with:
Open a new project and select 16:9 settings.
Import 16:9 clips from your camera or whatever source.
If you import 4:3 clips/pictures: enlarge en position these on the timeline so that the enlarged picture will fill the entire 16:9 screen.
Hope this helps a bit.