Updating in the Device View will only update the metadata.calibre cachefile which calibre uses to list the books on device.
In order to update the metadata in the Kobo database, do as GeoffR suggested. (you may need to set Preferences --> Sending Books to device --> Metadata Management == Automatic)
If you wish the cover to be in the book when you open it and page over to the beginining, you will need to resend the book.
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