how to modify contents in a nested xml

I need to modify the CompValue from "C02" to "C02,C03" . How to achieve this ?I have tried various ways to modify the XML content but so far could not find a solution.

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml("XML value");

XmlNode root = xmlDoc.DocumentElement;
XmlNode myNode = root.SelectSingleNode("descendant::CompValue");
myNode.Value = "blabla";

Also

root.SelectSingleNode("//CritGroup/Crit").InnerText = "NewValue";

This is my sample xml

<Search Name="Test1" ProjTypeID="107">
    <SearchType SearchTypeID="20246" />
    <FiscalYear Year="2022" />
    <CorpEnt CEID="367" CEName="Sample" />
    <CritGroup CritGroupID="1" CritGroupAndOr="AND">
        <Crit CritID="205" RelID="275" CompValue="C02" CompValueHuman="C02" AndOr="AND" TextBoxName="tbProjNum" /> 
        <Crit CritID="208" RelID="280" CompValue="11" CompValueHuman="Yes" AndOr="AND" TextBoxName="" />
    </CritGroup>
    <PageSize Size="200" />
</Search>

Please try the following solution.

It is using LINQ to XML API that is available in the .Net Framework since 2007.

c#

void Main()
{
	XDocument xdoc = XDocument.Parse(@"<Search Name='Test1' ProjTypeID='107'>
			<SearchType SearchTypeID='20246'/>
			<FiscalYear Year='2022'/>
			<CorpEnt CEID='367' CEName='Sample'/>
			<CritGroup CritGroupID='1' CritGroupAndOr='AND'>
				<Crit CritID='205' RelID='275' CompValue='C02' CompValueHuman='C02'
				      AndOr='AND' TextBoxName='tbProjNum'/>
				<Crit CritID='208' RelID='280' CompValue='11' CompValueHuman='Yes'
				      AndOr='AND' TextBoxName=''/>
			</CritGroup>
			<PageSize Size='200'/>
		</Search>");

	var Crits = xdoc.Descendants("Crit")
		.Where(x => x.Attribute("CompValue").Value == "C02");
	
	foreach (XElement Crit in Crits)
	{
		Crit.Attribute("CompValue").SetValue("C02,C03");
	}
}

Output XML

<Search Name="Test1" ProjTypeID="107">
  <SearchType SearchTypeID="20246" />
  <FiscalYear Year="2022" />
  <CorpEnt CEID="367" CEName="Sample" />
  <CritGroup CritGroupID="1" CritGroupAndOr="AND">
    <Crit CritID="205" RelID="275" CompValue="C02,C03" CompValueHuman="C02" AndOr="AND" TextBoxName="tbProjNum" />
    <Crit CritID="208" RelID="280" CompValue="11" CompValueHuman="Yes" AndOr="AND" TextBoxName="" />
  </CritGroup>
  <PageSize Size="200" />
</Search>

You need to get the complete node and replace the attribute. Here is the example

    var xmlDoc = new XmlDocument();
    xmlDoc.LoadXml(xml);

    var critNode = xmlDoc.SelectSingleNode("//Crit[@CompValue='C02']");

    if (critNode != null)
    {
        var compValueAttr = critNode.Attributes["CompValue"];
        if (compValueAttr != null)
        {
            compValueAttr.Value = "C02,C03";
        }
    }

    var modifiedXml = xmlDoc.OuterXml;
    Console.WriteLine(modifiedXml);