Write to XML node

Hi All;
I have a XML file like the old ini and I want to write to a specific node in it. I have the Microsoft sample (101 VB.NET Samples), it has a project with write and read XML files. Here it is the sample code from it.

VB Code:

Private Sub ModifyElementValue()
    ' Shows how to modify an element's text value
    Dim xDoc As New XmlDocument()
 
    xDoc.Load(mstrModifyFile)
 
    Me.txtXMLDisplay.Text = xDoc.OuterXml
 
    Dim xNode As XmlNode
    Dim xElem As XmlElement
 
    Dim xElmntFamily As XmlElement
    xNode = xDoc.SelectSingleNode("//Person")
 
    If Not (xNode Is Nothing) Then
        xElem = CType(xNode, XmlElement)
 
        ' Change "Gerald L. Smith" to "Jerry Smith"
        xElem.InnerText = "Jerry Smith"
 
        Me.txtXMLEdits.Text = xDoc.OuterXml
    Else
        Me.txtXMLEdits.Text = String.Format("Family Node was not found. Please try the '{0}' option first.", CMD_CREATE_XML)
    End If
End Sub

I run it and it didn't change the XML element with the new value, I have also another code and it dosen't work too.

VB Code:

Dim xmlList As XmlNodeList
    Dim xmlDoc As New XmlDocument
 
    Try
        xmlDoc.Load(XMLFileName)
 
        xmlList = xmlDoc.GetElementsByTagName(NodeName)
 
        If xmlList.Count > 0 Then
            xmlList(0).InnerText = ValueString
        End If
    Catch ex As XmlException
    End Try

Within the program when runing and debuging the value changed but in the real after I close the application and return to run it again the old value of the element still as it not changed :(