You can achieve it by code also, In you Global.asax file in Session_Start event write response.redirect to your start page like following.
void Session_Start(object sender, EventArgs e)
{
// Code that runs when a new session is started
Response.Redirect("~/Index.aspx");
}
You can get redirect page name from database or any other storage to change the application start page while application is running no need to edit web.config or change any IIS settings