CREATE TABLE t
(  
  a_id       NUMBER not null,
  t_id       NUMBER,
  c_name       VARCHAR2(100),
  d_order     NUMBER default 0,
  CONSTRAINT t_pk PRIMARY KEY (a_id, t_id)
);

INSERT INTO t
WITH cte1 (a_id, t_id, c_name, d_order) AS 
(
  SELECT 675150, 770650, 'name', 0 FROM dual UNION
  SELECT 675150, 780700, 'name', 0 FROM dual UNION
  SELECT 675150, 780701, 'name', 0 FROM dual UNION
  SELECT 675180, 770550, 'name', 0 FROM dual UNION
  SELECT 675180, 780800, 'name', 0 FROM dual UNION
  SELECT 675180, 780801, 'name', 0 FROM dual
)
SELECT * FROM cte1;

SELECT 
  t.a_id, 
  t.t_id, 
  t.d_order, 
ROW_NUMBER() OVER (PARTITION BY t.a_id 
                            ORDER BY t.a_id, t.t_id DESC) AS rn
FROM t;

A_ID	T_ID	D_ORDER	RN
675150	780701	0	1
675150	780700	0	2
675150	770650	0	3
675180	780801	0	1
675180	780800	0	2
675180	770550	0	3

SQL> select * from t;
      A_ID       T_ID C_NAME                  D_ORDER
---------- ---------- -------------------- ----------
    675150     770650 name                          0
    675150     780700 name                          0
    675150     780701 name                          0
    675180     770550 name                          0
    675180     780800 name                          0
    675180     780801 name                          0

6 rows selected.

SQL> merge into t t1
  2  using ( SELECT rowid, ROW_NUMBER() OVER (PARTITION BY t.a_id ORDER BY t.t_id DESC) AS rn
  3          FROM t ) t2
  4  on ( t1.rowid = t2.rowid )
  5  when matched then update set d_order = t2.rn
  6  /

6 rows merged.

SQL> select * from t order by t.a_id, t.t_id DESC;
      A_ID       T_ID C_NAME                  D_ORDER
---------- ---------- -------------------- ----------
    675150     780701 name                          1
    675150     780700 name                          2
    675150     770650 name                          3
    675180     780801 name                          1
    675180     780800 name                          2
    675180     770550 name                          3

6 rows selected.

SQL> rollback;

Rollback complete.

SQL> update t t1
  2  set d_order =
  3      ( select rn
  4        from ( select rowid rid, ROW_NUMBER() OVER (PARTITION BY t.a_id ORDER BY t.t_id DESC) AS rn
  5               from t ) t2
  6        where t2.rid = t1.rowid )
  7  /

6 rows updated.

SQL> select * from t order by t.a_id, t.t_id DESC;
      A_ID       T_ID C_NAME                  D_ORDER
---------- ---------- -------------------- ----------
    675150     780701 name                          1
    675150     780700 name                          2
    675150     770650 name                          3
    675180     780801 name                          1
    675180     780800 name                          2
    675180     770550 name                          3

6 rows selected.