public boolean update(Administrador admin){
        PreparedStatement ps;
        try {
            ps = mycon.prepareStatement(
                "UPDATE Administradores "+ 
                "SET passwd=?, dni=? "+ 
                "WHERE userName=?"
            );
            ps.setString(1, admin.getPassword());
            ps.setString(2, admin.getDNI());
            ps.setString(3, admin.getNombre());
            return (ps.executeUpdate()>0);
        } catch (SQLException ex) {
            Logger.getLogger(AdministradorCRUD.class.getName()).log(Level.SEVERE, null, ex);
        }
        return false;
    }