I get the impression you are talking about a Word Mail Merge, using your c# code to provide the fields for the merge.
Using the word interops (special dlls MS provides) you can perform this along the lines of:
Application word = new Word.Application();
word.Documents.Open ( "C:\sometemplate.doc" );
word.ActiveDocument.Select ( );
Range range = word.ActiveDocument.Range ( 0, 0 );
word.ActiveDocument.Fields.Add ( range, -1, "SET field 'value'", true );
word.ActiveDocument.Fields.Update ( );
word.ActiveDocument.Fields.Unlink ( );
word.ActiveDocument.Save ( );
word.ActiveDocument.Close ( );
I suggest that you do close the document and then use Process.Start()
to open it again as the Word interops can be a big buggy when it comes to memory handling.