How do you modify the web.config appSettings at runtime?

Question

I am confused on how to modify the web.config appSettings values at runtime. For example, I have this appSettings section:

Answer 1

Who likes directly to the point,

In your Config

    <appSettings>

    <add key="Conf_id" value="71" />

  </appSettings>

in your code(c#)

///SET
    ConfigurationManager.AppSettings.Set("Conf_id", "whateveryourvalue");
      ///GET              
    string conf = ConfigurationManager.AppSettings.Get("Conf_id").ToString();