how to modify web.config in asp.net using C#

You can modify web.config at run time. For exmaple to add a property in app settings section of web.config, you can use the following c# code

Configuration myConfiguration = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");
myConfiguration.AppSettings.Settings.Add("CommentsLimit", "10");
myConfiguration.Save();


To update the existing property:
Configuration myConfiguration = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");
myConfiguration.AppSettings.Settings["CommentsLimit"].Value = "20";
myConfiguration.Save();