The latest version of Drop Dead: Remains should only be available here as a download ("Drop Dead Collection.zip") after you've purchased the collection here. That's the only way of updating Drop Dead: Remains as far as I know.
If the user has bought Drop Dead Collection then the download link should be at least on the email that contains all download links for purchased games. That email can be requested from itch.io. It's kinda difficult, I know, but that's how itch.io works, I guess...
