var
WordApp,Doc,R:OleVariant;
b:integer;
bk,bk_name, bk_value:string;
docname:string;

procedure ChangeBookmark;
begin
docname:='C:\MyDoc.doc';
try
  WordApp.Documents.Open(docname);
except
 exit;
end;

try
 if WordApp.ActiveDocument.fullname=docname  then
    doc:=WordApp.ActiveDocument
  else  exit;
 except
  exit;
 end;

 for b:=1 to doc.Bookmarks.Count do
   begin
    bk:=doc.Bookmarks.Item(b).Name;    
    if bk=bk_name then
       begin
             R:=doc.Bookmarks.Item(b).Range;
             R.Text:=bk_value;
             break;
        end;      
   end;

end;