<?xml version="1.0" encoding="utf-8"?>
<Rules>
  <Node 转换率="1" />
</Rules>

                XmlDocument xmldoc = new XmlDocument();
                xmldoc.Load("***.xml");
                XmlNode root = xmldoc.SelectSingleNode("Rules");

                ((XmlElement)root.SelectSingleNode("Node")).SetAttribute("转换率", "1");
                xmldoc.Save("***.xml");