If you know what the filename is for the old file, you can just use a FileSystemObject like so:
set fs = CreateObject("Scripting.FileSystemObject")
' do whatever you need to here using the FileSystemObject methods
set fs = Nothing
and rename the file, then save the new one with the old name. You can read up on what the FileSystemObject methods are on the Microsoft site, or use a search engine to locate the documentation you like better.