how to save my changes in web.config

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C#

ASP.NET

hi i want change aouthentication mode in my project from code dinamically but cant save my change in web.config
my code for change is:

Configuration config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/HR");
       
        System.Web.Configuration.SystemWebSectionGroup systemWeb = (System.Web.Configuration.SystemWebSectionGroup)config.GetSectionGroup("system.web");
        System.Web.Configuration.AuthenticationSection section = (System.Web.Configuration.AuthenticationSection)systemWeb.Sections["authentication"];

        System.Web.Configuration.AuthenticationMode currentMode = section.Mode;
        section.Mode = System.Web.Configuration.AuthenticationMode.Windows;
if (!section.ElementInformation.IsLocked || !section.IsReadOnly())
        config.Save();

and error is:
ConfigurationSection properties cannot be edited when locked.

please help me