If you want to save the image as a single image, you need to download it while creating it. Once you exit outpainting mode, you will no longer be able to access the complete image (unless you manually stitch the generated frames together). This is because the generated frames are stored separately, without the rest of the larger composition.
If you want to download the artwork as a single image while creating it, simply click the download icon in the upper right corner. It looks like a downward arrow with a horizontal line underneath.