public void update(string userName, string password, string lastName, string firstName,string address, int phoneNumber, string validUser)
        {
            con.Open();
            OleDbCommand cmd = con.CreateCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "update users set username='" +userName+ "',password='"+password+"',lastname='"+lastName+"',firstname='"+firstName+"',address='"+address+"',phonenumber='"+phoneNumber+"' where username='"+validUser+"'";
            cmd.ExecuteNonQuery();
            con.Close();
        }