How to edit the list of files to download in a .torrent file?

I have a torrent that contains many files in it, but I don't want to download all of them because I need only few files among them. I am not downloading from a torrent client, but I am actually using put.io service that converts torrents into direct downloads for me.

I want to edit the .torrent file (or magnet link) in such a way that I get only those files which I need from the torrent. I tried by editing with text editors, but I am getting some crazy hash errors when I did that.

I have seen people editing trackers in a torrent file (using online editors and other editors), but none of them edited the list of files in the torrent file.

So, the question is:

How to edit a .torrent file (and save it back as another .torrent file) such that when you run that torrent file, it only downloads only those files you need?

It may not be possible - from some preliminary googling

"The torrent file format makes adding or removing files impossible without access to the original files. " from Faq - Torrent Editor

This might need client support to only download the needed files, however put.io doesnt seem to support it yet