See code below.

Arne

PS: I think it is a very bad replacement !

===========================================

using System;
using System.Xml;

namespace E
{
    public class Program
    {
        public static void Modify(XmlDocument doc, int val1, int val2,
int val3)
        {
            XmlElement elm =
(XmlElement)doc.SelectSingleNode("//details/detail[t1[1]='" + val1 + "'
and t1[2]='" + val2 + "' and t1[3]='" + val3 + "']");
            elm.RemoveChild(elm.SelectSingleNode("t1[text()='" + val1 +
"']"));
            elm.RemoveChild(elm.SelectSingleNode("t1[text()='" + val2 +
"']"));
            elm.RemoveChild(elm.SelectSingleNode("t1[text()='" + val3 +
"']"));
            XmlElement t1 = doc.CreateElement("t1");
            t1.AppendChild(doc.CreateTextNode(val1 + " or " + val2 + "
or " + val3));
            elm.AppendChild(t1);
        }
        public static void Main(string[] args)
        {
            string xml = @"<?xml version='1.0'?>
<details>
<detail>
  <description>name</description>
  <t1>10</t1>
  <t1>100</t1>
  <t1>1000</t1>
  </detail>
<detail>
  <description>name</description>
  <t1>11</t1>
  <t1>111</t1>
  <t1>11111</t1>
  </detail>
<detail>
  <description>name</description>
  <t1>21</t1>
  <t1>221</t1>
  <t1>2221</t1>
  </detail>
</details>";
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(xml);
            Modify(doc, 10, 100, 1000);
            doc.Save(Console.Out);
            Console.ReadKey();
        }
    }
}