XmlDocument xml = new XmlDocument();
            xml.Load("1.xml");

            XmlNodeList xl = xml.SelectNodes(@"A/B/C");

            bool bfound = false;
            XmlNode xnfd = null;
            foreach (XmlNode xn in xl)
            {
                if(xn.InnerText.Equals("c"))
                {
                    bfound = true;
                    xnfd = xn;
                    break;
                }

            }

            if(bfound)
            {
                xnfd.InnerText = "newc";
                xml.Save("1.xml");
            }
            else
            {
                //没有，加新的
            }

------解决方案--------------------
C# code

            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load("XMLFile1.xml");
            var elmtsB = xmlDoc.SelectNodes("//B");
            foreach (XmlNode elB in elmtsB)
            {
                foreach (XmlNode el in elB.ChildNodes)
                {
                    if (el.Name == "C")
                    {
                        Console.WriteLine("It has C");
                        break;
                    }                    
                }                 
            }