Assumption is that the user downloads the document first, then makes changes to that document and it should replace the older version. When you edit a filedocument you can use the upload functionality to replace the document. So give the user first a download button of that file document. Create a replace button where you then just show a dataview of that same file document with the upload functionality and give him the abi lity to save that filedocument again.
Regards,
Ronald