Changing the text inset's source file with a script

I'm looking for a way to replace an existing text-inset in a document with the contents of an other file (by reference), by means of scripting. Say, I have a document c:\referer.fm, in which there is a text inset of which the source file is: c:\A.fm

I also have c:\B.fm, and now I want to change referer.fm so that no longer the text inset holds the text from A.fm, but from B.fm.

I tried this:

var doc = app.ActiveDoc;

var textinset = doc.FirstTiInDoc;

var pathtosource = textinset.TiFile;

pathtosource = "c:\B.fm";

textinset.UpdateTextInset();

But this doesn't work. I guess setting 'pathtosource' cannot be done this way. If anyone knows the way to do this, I would really appreciate your help!

David