请参阅以下链接

How to edit a value in root element of XML file

<Model name="Test.cre" version="v2.0" unit="m" count="0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

Here Model is the root element I want to update the count attribute in root element using C#.

How to do this using C#

           XmlDocument xmlDoc = new XmlDocument();
           xmlDoc.Load(@"C:\Users\KK\Desktop\Sample.xml");

XmlElement node1 = xmlDoc.SelectSingleNode("/root") as XmlElement;
           if (node1 != null)
           {

               node1.SetAttribute("count", nodeCount.ToString()); // if you want an attribute

           }

           xmlDoc.Save(@"C:\Users\KK\Desktop\Sample.xml");

Please refer the following Link