Change web.config in Run Time

  protected void Button2_Click(object sender, EventArgs e)

    {

                TextBox3.Text = System.Web.Configuration.WebConfigurationManager.ConnectionStrings["myDatabaseName"].ConnectionString;

        Configuration mycon = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");

        mycon.ConnectionStrings.ConnectionStrings["myDatabaseName"].ConnectionString = "New Database Name";

        mycon.AppSettings.Settings["myKey"].Value = "jey";

        mycon.Save();

    }

<connectionStrings>

        <add name="myDatabaseName" connectionString="New Database Name" />

connectionStrings>

   <appSettings>

      <add key="myKey" value="jey" />

    appSettings>