For your example, you can simply add an inline conditional statement for the icon.name property. Or for more advanced use especially if you intend to add more changes, you can use States.

Here's a sample one for the inline:

icon.name: switch.checked ? "tick" : "close"

I can recommend this one.

could you be so kind an give me a example

beacuse how to define a variable in qml code:

• add the icon

here is the default Main.qml
when create a new project.

import QtQuick 2.7
import QtQuick.Controls 2.2
import Lomiri.Components 1.3
import QtQuick.Layouts 1.3
import Qt.labs.settings 1.0

import Greeter 1.0

ApplicationWindow {
    id: root
    objectName: 'mainView'

    width: units.gu(45)
    height: units.gu(75)
    visible: true

    Greeter {
        id: greeter
        name: "Rust + Ubuntu Touch"
    }

    Page {
        anchors.fill: parent

        header: PageHeader {
            id: header
            title: i18n.tr('bla')
        }

            Icon {
                width: units.gu(5)
                height: units.gu(5)
                name: 'cancel'
                
            }

        ColumnLayout {
            spacing: units.gu(2)
            anchors {
                margins: units.gu(2)
                top: header.bottom
                left: parent.left
                right: parent.right
                bottom: parent.bottom
            }

            Item {
                Layout.fillHeight: true
            }

            Label {
                id: label
                text: i18n.tr('Press the button below!')
            }

            Button {
                text: i18n.tr('Compute greeting')
                onClicked: {
                    label.text = greeter.compute_greetings("Hello, ");
                }
            }

            Switch {
                checked: off
                onClicked: {
                        label.text = greeter.compute_greetings("sw on");
                }

            }


            Item {
                Layout.fillHeight: true
            }
        }
    }
}