ConfigModificatorUsage.cs

string appConfigFilePath = string.Concat(Assembly.GetExecutingAssembly().Location, ".config");
ConfigModificatorSettings appConfigWriterSettings =
    new ConfigModificatorSettings("//appSettings", "//add[@key='{0}']", appConfigFilePath);

string value = ConfigurationManager.AppSettings["testKey1"];
System.Console.WriteLine("Value before modification: {0}", value);

ConfigModificator.ChangeValueByKey(
    key: "testKey1",
    value: "ChangedValueByModificator",
    attributeForChange: "value",
    configWriterSettings: appConfigWriterSettings);

ConfigModificator.RefreshAppSettings();
value = ConfigurationManager.AppSettings["testKey1"];
System.Console.WriteLine("Value after modification: {0}", value);

//Example how to change Custom XML configuration
string carsConfigFilePath = "Cars.xml";
ConfigModificatorSettings carsConfigWriterSettings =
    new ConfigModificatorSettings("//cars", "//car[@name='{0}']", carsConfigFilePath);

ConfigModificator.ChangeValueByKey(
    key: "BMW",
    value: "Mazda",
    attributeForChange: "name",
    configWriterSettings: carsConfigWriterSettings);