how to get the specific node's value

I am trying to update some of xml's nodes. But I am not very experienced in XML stuff. I read some examples but I am doing something wrong

here is my xml...

<?xml version="1.0" encoding="ISO-8859-9"?>
<BANK_VOUCHERS>
    <BANK_VOUCHER DBOP="INS" >
       <DATE>30.12.2014</DATE>
       <NUMBER>123</NUMBER>
       <TYPE>3</TYPE>
       <TRANSACTIONS>
           <TRANSACTION>
               <TYPE>1</TYPE>
               <BANKACC_CODE>BANK1 TL-HESAP</BANKACC_CODE>
               <ARP_CODE>CARI1_NR1</ARP_CODE>

I am trying to REPLACE the ARP_CODE value but in some of the xml files, there might not be any ARP_CODE node.

my code is below but it is not working. what am I doing wrong?

XDocument xdoc = XDocument.Load(filename);
foreach (var element in xdoc.Elements("ARP_CODE"))
     element.Value = "foo";    
xdoc.Save(filename.Substring(0, filename.LastIndexOf('.')) + "_upd.xml");