I need help to update XML file

See more:

VB

XML

What I am trying to do is to use a variable in vb code to update a xml configfile's <LastRunDateTime>2018-03-30T00:00:00</LastRunDateTime> every time the program runs the strSQL.
I want to only update the LastRunDateTime node text

Here is the config xml file:

<?xml version="1.0" encoding="UTF-8"?>
<DocGetAuditReport.Config>
	<LastRunDateTime>2018-03-30T00:00:00</LastRunDateTime>
</DocGetAuditReport.Config>

What I have tried:

 Public Class MyClassR
    Shared Sub main()
        Dim strLastRunDateTime As String
		Dim dtmCurrentDateTime As Date = Now
        Dim objXmlConfigDoc As XmlDocument
        Dim strSQL As String
        Dim objXmlResultDoc As XmlDocument
        Dim objXMLNode As XmlNode
        Try
           'Load DocGetAuditReport file and get last run date time dtmLastRunDateTime
            'Dim objXmlTaskConfigDoc As XmlDocument = Msc.Integration.Utility.Library.v4.ProgramExecution.GetConfigFile("Msc.Integration.MessageWarehouse.Task.v4.DocGetAuditReport.Config.xml")
			 Dim objXmlTaskConfigDoc As XmlDocument = GetConfigFile("Config.xml")
            strLastRunDateTime = objXmlTaskConfigDoc.DocumentElement.SelectSingleNode("LastRunDateTime").InnerText.Replace("T", " ")
            strSQL = GetSql(strLastRunDateTime, dtmCurrentDateTime)

            'THIS IS WHERE I would like to use variable dtmCurrentDateTime to update objXmlConfigDoc xml file with current date instead of what it has now <LastRunDateTime>2018-03-30T00:00:00</LastRunDateTime>
            'objXmlConfigDoc = GetConfigFile("Config.xml")
    End Sub
    Private Shared Function GetSql(ByVal astrLastRunDateTime As String, ByVal adtmCurrentDateTime As Date) As String
        Dim strSQL As String
        strSQL ="SELECT Something, ProcessDateTime AS 'RequestDateTime'," + vbCrLf +
        "FROM TableName" + vbCrLf +
       "And (ProcessDateTime > CONVERT(DATETIME, '" + astrLastRunDateTime + "', 102))" + vbCrLf +
     "AND (ProcessDateTime <= CONVERT(DATETIME, '" + adtmCurrentDateTime.ToString + "', 102))" + vbCrLf +
       Return strSQL
    End Function
End Class