Sometimes you want to change the area selected in a sub image. You can do this by removing it from the sub image list and selecting the area again.
Alternatively or you can use the substitute command, which is S on your keyboard or the 
icon. The icon shows one image being replaced by another from the main scan area.
To use the substitute command simply highlight the sub image (in the column of sub image on the right of the screen) so that its frame turns red. Its selection rectangle will appear in the main scanned image. Now change the selection rectangle as you want, then hit the S key or the icon shown above.