using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Xml;
using System.Xml.Linq;
 
public class Test
{
        public static void Main()
        {
                string xml = @"<Table1>
                                 <buyer_id>0</buyer_id>
                                 <buyername>CompanyA</buyername>
                                 <address1>123 Simpsons Dr.</address1>
                                 <address2/>
                                 <city>Springfield</city>
                                 <state>ST</state>
                                 <postalcode>12345</postalcode>
                                 <eaddress/>
                                 <phone/>
                                 <fax/>
                                 <buyer_id>1</buyer_id>
                                 <buyername>CompanyB</buyername>
                                 <address1>456 Simpsons Dr.</address1>
                                 <address2/>
                                 <city>Springfield</city>
                                 <state>ST</state>
                                 <postalcode>12345</postalcode>
                                 <eaddress/>
                                 <phone/>
                                 <fax/>
                               </Table1>";
 
 
                 XDocument doc = XDocument.Parse(xml); //replace with xml file path
                 IEnumerable<XElement> buyersList = doc.Descendants("Table1"); //get the table node.
                 var ele = (from buyer in buyersList
                            where buyer.Element("buyername").Value == "CompanyA"
                            select buyer).SingleOrDefault(); 
                 ele.SetElementValue("address1", "SomeInfo");
                 ele.SetElementValue("address2", "SomeInfo");
 
                 Console.WriteLine(ele.Element("address2").Value);
        }       
}

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