Updating a table for a series of number by PL/SQL in oracle Database

DECLARE
   CURSOR gdwn
   IS
      SELECT *
        FROM godown; --(Table name) 
BEGIN
   FOR c1 IN gdwn
   LOOP
      UPDATE godown
         SET tp_id = (SELECT NVL (MAX (tp_id), 0) + 1
                        FROM godown)
       WHERE godown_id = c1.godown_id;

      COMMIT;
   END LOOP;
END;