Need to update id column using row_number()

I have table with user_id and user_name columns. I have to another column as ID. I want to populate the ID as below.

First it needs to find the unique user_id and its number of occurrences and then assign 1 to n number based on the occurrences.

I am able to get the output by using row_number()

select row_number() over (partition by user_id order by user_id) as seq ,user_id,user_name from test_csv

How can I update ID with seq coming out of above query.

Use a MERGE statement and correlate on the ROWID pseudo-column:

MERGE INTO test_csv dst
USING (
  SELECT row_number() over (partition by user_id order by user_id) as seq 
  FROM   test_csv
) src
ON (src.ROWID = dst.ROWID)
WHEN MATCHED THEN
  UPDATE
  SET id = seq;

or a correlated UPDATE:

UPDATE test_csv dst
SET id = (
           SELECT seq
           FROM   (
             SELECT row_number() over (partition by user_id order by user_id) as seq 
             FROM   test_csv
           ) src
           WHERE src.ROWID = dst.ROWID  
         );

Which, for the sample data:

CREATE TABLE test_csv (User_id, User_name, ID) AS
SELECT 123, 'ABC', CAST(NULL AS NUMBER) FROM DUAL UNION ALL
SELECT 456, 'BCD', NULL FROM DUAL UNION ALL
SELECT 123, 'ABC', NULL FROM DUAL UNION ALL
SELECT 123, 'ABC', NULL FROM DUAL UNION ALL
SELECT 234, 'XYZ', NULL FROM DUAL;

Then, after the MERGE or UPDATE, the table contains:

fiddle