The only place to selectively download file revisions appears to be under Unity Dashboard > Collaborate > Assets, otherwise you must use the "Go back to..." feature in Unity to restore the entire project to a revision.
Unfortunately, deleted or renamed files don't show up in the Assets view, meaning the only way to resurrect these files is to roll the entire project back to a revision containing them, make local copies, then restore the entire project back to the latest revision.
I recently had to do this to retrieve just a few lines of code and the back and forth syncing took nearly 30 minutes.
What would be great is if the same download link that appears next to files in the Assets view also appeared next to files in the Timeline view. This would be an intuitive way to browse previous revisions and cherry pick files from the past, whether they're deleted, renamed, or otherwise.