How to fetch an attribute value from xml using powershell and replace with string content

PowerShell has a nice feature called Member Enumeration which might come at hand when reading properties:

$xmldata.abc.myTask.task |Get-Member

   TypeName: System.Xml.XmlElement

Name                 MemberType            Definition
----                 ----------            ----------
ToString             CodeMethod            static string XmlNode(psobject instance)
...
WriteTo              Method                void WriteTo(System.Xml.XmlWriter w)
Item                 ParameterizedProperty System.Xml.XmlElement Item(string name) {get;}, System.Xml.XmlElement Item(string localname, string ns) {get;}
#cdata-section       Property              string #cdata-section {get;set;}

But it might get confusing when you want to write data because you will need to be specific in the node branch you want to write (as it would be incorrect to write to all branches in once).

In this specific case the branche is at the $xmldata.abc.myTask:

                     ┌─ [0].task.'#cdata-section'
$xmldata.abc.myTask ─┤
                     └─ [1].task.'#cdata-section'

So you will need to define whether you want to change the first or the second mytask. In case you want to replace the first mytask (in the zero based array):

$xmldata.abc.myTask[0].task.'#cdata-section' = $StrContent
$xmldata.Save([Console]::Out)

<?xml version="1.0" encoding="ibm850"?>
<abc>
  <myTask id="MyBatchExport" name="MyBatchExport" scriptFormat="javascript">
    <task><![CDATA[my own content to replace in between CDATA]]></task>
  </myTask>
  <myTask id="MyBatchImport" name="MyBatchImport" scriptFormat="javascript">
    <task><![CDATA[var requestItemsParm= execution.getVariable("requestItems");
      var requestItemsParm= JSON.parse(requestItemsParm);
      var batchImport = {};]]></task>
  </myTask>
</abc>

To resolve a set of nodes based on some condition relating to their parent nodes, XPath is usually the right tool for the job.

For example, to get the CDATA section under only those <task> nodes whose parents are <myTask> with a name attribute value of MyBatchExport, you could use the following expression:

$targetNodes = $xmldata.SelectNodes('//myTask[@name = "MyBatchExport"]/task/text()')

Then simply loop over each resolved node and update the contents:

$targetTaskNodes |ForEach-Object { $_.InnerText = $StrContent }