Sharepoint Infopath Form 2013 Change Event not updating the value back in the form

I have an infopath form .On the change event of To Field i am updating the value of another field called Shift. For that i have written following code.

public void To_Changed(object sender, XmlEventArgs e)
    {
        // Write your code here to change the main data source.
        ///my:myFields/my:RequestDetails/my:IndividualRequest/my:To
        // Write your code here.
        // Write your code here to change the main data source.
        ///my:myFields/my:RequestDetails/my:IndividualRequest/my:To
        try
        {
            XPathNavigator xmlDoc = this.MainDataSource.CreateNavigator();
            XPathNavigator xTo = xmlDoc.SelectSingleNode("my:myFields/my:RequestDetails/my:IndividualRequest/my:To", this.NamespaceManager);
            XPathNavigator xShift = xmlDoc.SelectSingleNode("/my:myFields/my:RequestDetails/my:IndividualRequest/my:Shifts", this.NamespaceManager);



            xShift.SetValue("6");

        }
        catch (Exception ex)
        {
            Log.WriteLog(ex);

        }
    }  

I have published the form and uploaded the form as contenttype. however when i change the To value in the infpath form, Shift Field value not updating.