Look at it more from the perspective of how you can save the file. When you open the file, it has been transferred to your computer already, and then you can do whatever you want with it, but if the path to the file is not located on a local drive, or a network share, then you'll have to save the file locally, and then transfer it back.
There are a number of easy methods to ftp a file using code.

Another way would be to have a page with a simple editor on that same site. But then you get into issues with permissions.

Matt