XElement xmlTree = new XElement("Root",
    new XElement("Child1", "child1 content"),
    new XElement("Child2", "child2 content"),
    new XElement("Child3", "child3 content"),
    new XElement("Child4", "child4 content"),
    new XElement("Child5", "child5 content")
);
XElement child3 = xmlTree.Element("Child3");
child3.ReplaceWith(
    new XElement("NewChild", "new content")
);
Console.WriteLine(xmlTree);

Dim xmlTree As XElement = _ 
        <Root>
            <Child1>child1 content</Child1>
            <Child2>child2 content</Child2>
            <Child3>child3 content</Child3>
            <Child4>child4 content</Child4>
            <Child5>child5 content</Child5>
        </Root>

Dim child3 As XElement = xmlTree.<Child3>(0)
child3.ReplaceWith(<NewChild>new content</NewChild>)
Console.WriteLine(xmlTree)