For that add a CheckBox say chkRemember. And replace the following line
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1, Login1.UserName, DateTime.Now, DateTime.Now.AddMinutes(2880), Login1.RememberMeSet, roles, FormsAuthentication.FormsCookiePath);
with
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1, Login1.UserName, DateTime.Now, DateTime.Now.AddMinutes(2880), chkRemember.Checked, roles, FormsAuthentication.FormsCookiePath);