Change the server setting in the configuration from testhost.somecompany.com to productionhost.somecompany.com.

XML file:

    <?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
        <add key="server" value="testhost.somecompany.com" />
    </appSettings>
</configuration>

Cake Task:

    Task("Transform")
    .Does(() =>
{
    var file = File("test.xml");
    XmlPoke(file, "/configuration/appSettings/add[@key = 'server']/@value", "productionhost.somecompany.com");
});

Modify the noNamespaceSchemaLocation in an XML file.

XML file:

    <?xml version="1.0" encoding="utf-8" ?>
<Commands xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="Path Value">
</Commands>

Cake Task:

    Task("Transform")
    .Does(() =>
{
    var file = File("test.xml");
    XmlPoke(file, "/Commands/@xsi:noNamespaceSchemaLocation", "d:\\Commands.xsd", new XmlPokeSettings {
        Namespaces = new Dictionary<string, string> {
            { /* Prefix */ "xsi", /* URI */ "http://www.w3.org/2001/XMLSchema-instance" }
        }
    });
});

Remove an app setting from a config file.

XML file:

    <?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
        <add key="server" value="testhost.somecompany.com" />
        <add key="testing" value="true" />
    </appSettings>
</configuration>

Cake Task:

    Task("Transform")
    .Does(() =>
{
    var file = File("test.xml");
    XmlPoke(file, "/configuration/appSettings/add[@testing]", null);
});