The easiest way to replace images in your SmartAlbums project would be to drag your new image files into the original image source folder that was used to import your images into SmartAlbums. When you do this, choose to overwrite the old images with the newly added ones. As long as the images in both folders have exactly the same filenames, the new images will automatically replace the old ones if the project is open when you make the switch:

If your files have been overwritten in the source folder, but they haven't updated in SmartAlbums automatically, you can update the files manually by selecting all of the images in the middle-Timeline area and choosing Reload from Original:

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