Not sure, what you want to achieve, but take a look at below code:
Dim xcontent As String = "<VDZ>
<Transformer>
<Tx>
<Handle>1</Handle>
<ID>TBA</ID>
<Rating>200kVA</Rating>
<Z>0.03444</Z>
<Points>
<P ID='0' X='120' Y='96' />
</Points>
</Tx>
</Transformer>
</VDZ>"
Dim xdoc AS XDocument = XDocument.Parse(xcontent)
Dim secondTransformer As XElement = New XElement(xdoc.Descendants("Transformer").FirstOrDefault())
Dim h As XElement = secondTransformer.Descendants("Handle").FirstOrDefault()
h.Value = 2
xdoc.Root.Add(secondTransformer)