[Schwarzenegger]

I have my ebooks organised in Calibre on my computer. I copy my ebooks to Dropbox. I always sync Dropbox with Calibre. Marvin on my ipad connects with Dropbox.
How this whole thing works is still a mystery to me. I got Marvin to read Dropbox by pushing dozens of buttons dozens of times and suddenly Hey presto there were my ebooks, tags and all. (I exaggerate, but not much).
Changing the tag of a book in Dropbox does not seem to change the tag on the Marvin copy.
There are 2 occasions where I want to delete an ebook from Marvin:
1 I don't like the book I am reading
2 I have changed the tag of a book in Calibre. In this case I want to simply replace the the book existing in Marvin with the newly tagged book on Dropbox.

How do I delete an ebook from Marvin?

Cheers,