When importing, ReadCube Papers may tag a supplement as an article or vice versa. To correct this, in your library view - click on the file icon and you'll see thumbnails of the files associated to the paper. Click on the More button to choose if you need to make a supplement the main article.

 

You can even remove linking of the supplements if they aren't correct. Please note - you cannot do this when article is open, only from library view.