Change authentication Timeout value programmatically C#

Here is my web config file

<authentication mode="Forms">
  <forms loginUrl="~/Auth/signin.aspx" timeout="1" slidingExpiration="true" />
</authentication>

I tried following way

   Configuration conn = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath);
            AuthenticationSection section = (AuthenticationSection)conn.SectionGroups.Get("system.web").Sections.Get("authentication");
            FormsAuthenticationConfiguration currentForms = section.Forms;
            var tim = currentForms.Timeout;
            currentForms.Timeout = new TimeSpan(0, 2, 0);
            int timeout = currentForms.Timeout.Minutes;

But it doesn't change in web config file. Can you please help me to Change authentication Timeout value in C# code.