Smart collections are really a filter on all of your images, they aren't stored as a set of specific images. So if you want to add or delete images, you need to use an ordinary collection, not a smart one.
Please refer the below screenshot to delete the collections under smart collection.
For adding the image to the collection, Drag photos from the Grid view to a collection in the Collections panel.
Hope that helps.
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You can't add photos to a smart collection by drag and drop. The photos must match the criteria set up for that smart collection.
Thank you so much!!