Update a column with sequential number

To get sequence number use the below query.

select ROW_NUMBER() OVER (ORDER BY COLUMN_NAME) AS seq,A.* from TABLE_NAME A

To Update sequential number in column

MERGE INTO TABLE_NAME t
USING (
SELECT ROWID AS rid,
ROW_NUMBER() OVER (ORDER BY COLUMN_NAME) AS seq
FROM TABLE_NAME
) s
ON (t.ROWID = s.rid)
WHEN MATCHED THEN
UPDATE SET t.COLUMN_NAME= s.seq;