Can't modify app.config file with no error

Question

I'm struggling with trying to modify the appSettings part of the app.config file of my application. However I'm not receiving any error/exception and I can read values without any problem. Here is how I tried to set values :

ConfigurationManager.AppSettings.Set("DaysLeft", Days.Text.ToString());
//    
System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.AppSettings.Settings["DaysLeft"].Value = Days.Text.ToString();
//when I debug the app, I notice that the value is changed in the previous line

The app.config file isn't being modified.

Answer 1

System.Configuration.Configuration config = 
         ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.AppSettings.Settings["DaysLeft"].Value = Days.Text.ToString();
config.Save(); 

Answer 2

You need to save your changes

config.Save(ConfigurationSaveMode.Modified);

Also, if you want to reload the app.config

ConfigurationManager.RefreshSection("appSettings");