ChangeModificator.cs

public static void ChangeValueByKey(string key, string value, string attributeForChange, ConfigModificatorSettings configWriterSettings)
{
    XmlDocument doc = ConfigModificator.LoadConfigDocument(configWriterSettings.ConfigPath);
    XmlNode rootNode = doc.SelectSingleNode(configWriterSettings.RootNode);

    if (rootNode == null)
    {
        throw new InvalidOperationException("the root node section not found in config file.");
    }
    try
    {
        XmlElement elem = (XmlElement)rootNode.SelectSingleNode(string.Format(configWriterSettings.NodeForEdit, key));
        elem.SetAttribute(attributeForChange, value);
        doc.Save(configWriterSettings.ConfigPath);
    }
    catch (Exception ex)
    {
        throw ex;
    }
}

private static XmlDocument LoadConfigDocument(string configFilePath)
{
    XmlDocument doc = null;
    try
    {
        doc = new XmlDocument();
        doc.Load(configFilePath);
        return doc;
    }
    catch (System.IO.FileNotFoundException e)
    {
        throw new Exception("No configuration file found.", e);
    }
}