It's not as obvious as it should be. Navigate to the commit that changed the file. Press and hold the pencil icon in the file's header widget. You should see a menu pop up with options to edit the new and old revisions. The old revision is the file as it existed before the change, and new is the file after the change. Unfortunately, as there is no "Save As", you would have to resort to copy/paste to save.
The other option is to switch to the tree view. That shows arbitrary files as they existed in a given revision. If you want to get a file's contents before the change, you have to switch to the parent commit.