How to change a value of an attribute?

in the last days i tried to change a value of a single attribute of this application File

The Xml-File:

<?xml version="1.0" encoding="utf-8"?>
  <asmv1:assembly xmlns:asmv1="urn:schemas-microsoft-com:asm.v1"
  xmlns:asmv2="urn:schemas-microsoft-com:asm.v2">
    <assemblyIdentity name="Nameexample" version="2.3.4.5" publicKeyToken="=0123456789" language="neutral" processorArchitecture="x86" xmlns="urn:schemas-microsoft-com:asm.v1" />
    <description asmv2:publisher="PublisherExample" asmv2:product="Productexample" asmv2:supportUrl="SupportExample" xmlns="urn:schemas-microsoft-com:asm.v1" />
      <deployment install="true" mapFileExtensions="true" minimumRequiredVersion="2.3.4.5" trustURLParameters="true">
           <subscription>
              <update>
                <beforeApplicationStartup />
              </update>
            </subscription>
          <deploymentProvider codebase="http://Test" />
      </deployment>
  </asmv1:assembly>

Here i try to change the value of

<description asmv2:product = "Productexample">

into

<description asmv2:product = "Productexample2">

,and the value of

<deploymentProvider codebase="http://Test" />

into

<deploymentProvider codebase="http://Test2" />

Currently i tried :

private void changeAttribute (string xmlPath)
{
    string newValue = "Productexample2";
    XmlDocument xmlDoc= new XmlDocument();

    xmlDoc.Load(xmlPath);

    XmlNode node = xmlDoc.SelectSingleNode("asmv1:assembly/description/asmv2:product");
    node.Attributes[0].Value = newValue;

    xmlDoc.Save(xmlPath);
}

But it throws the Exception 'An unhandled exception of type 'System.Xml.XPath.XPathException' occurred in System.Xml.dll' so i think asmv1:assembly/description/asmv2:product is wrong ...

any suggestions of code?

as always you can correct me in any way :)