If you want to save your outpainting as a single image, you need to download it at the time of creation. Once you exit outpainting mode, you will not be able to access the full image again (unless you stitch the generation frames together manually). This is because generation frames are stored individually, without the rest of the larger composition.

If you want download your outpainting as a single image whilst creating, just click the download icon in the top-right hand corner. This looks like a downward arrow with a horizontal line under it.