Change value of a Specific Property in app.config using C#

Question

I want to change the value of <level key="LoggingLevel" value="Test" /> from my app.config file using C#.

App.Config

 

<?xml version="1.0" encoding="utf-8" ?>

<configuration>

<configSections>

<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />

</configSections>

<log4net>

<appender name="FileAppender" type="log4net.Appender.FileAppender">

<file value="C:\dev\log-file.txt" />

<appendToFile value="true" />

<layout type="log4net.Layout.PatternLayout">

<conversionPattern value="Level - %level%newline" />

</layout>

</appender>

<root>

<level key="LoggingLevel" value="Test" />

<appender-ref ref="FileAppender" />

</root>

</log4net>

<startup>

<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2" />

</startup>

</configuration>

 

I want to change the value from Test to Debug using C#. How can I do it?
Thanks in advance. :)