Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE table_name ( product, "GROUP", seq ) AS
SELECT 10,  5, 1 FROM DUAL UNION ALL
SELECT 10,  1, 5 FROM DUAL UNION ALL
SELECT 11,  2, 3 FROM DUAL UNION ALL
SELECT 11,  4, 4 FROM DUAL UNION ALL
SELECT 11, 24, 5 FROM DUAL
5 rows affected

MERGE INTO table_name dst
USING (
  SELECT ROWID AS rid,
         ROW_NUMBER() OVER (
           PARTITION BY product
           ORDER BY ROWNUM
         ) AS newSeq
  FROM   table_name
) src
ON ( dst.ROWID = src.rid )
WHEN MATCHED THEN
  UPDATE SET seq = src.newSeq;
5 rows affected

SELECT * FROM table_name


PRODUCT
GROUP
SEQ


10
5
1

10
1
2

11
2
1

11
4
2

11
24
3