SQL Server 2019

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CREATE TABLE table1 (
  [user] INT,
  line_number INT,
  header INT,
  Indexi INT
);

INSERT INTO table1
  ([user], line_number, header, Indexi)
VALUES
  ('290045', '0', '0', '3122411'),
  ('290045', '1', '0', '85'),
  ('290045', '2', '0', '833'),
  ('8733', '0', '0', '66')
  ;
4 rows affected

UPDATE table1

SET indexi = 500
FROM table1 t1 INNER JOIN (SELECT Indexi,[user], COUNT(*) cnt1 FROM table1 GROUP BY Indexi,[user]) t2
ON t1.Indexi = t2.Indexi AND t1.[user] = t2.[user]
WHERE (t1.[user] = 290045 AND t1.Indexi = 3122411 and cnt1 = 1) 
AND EXISTS ( SELECT 1 FROM table1 t1 WHERE t1.[user] = 290045 AND t1.Indexi = 3122411)
1 rows affected

SELECT * FROM table1


user
line_number
header
Indexi


290045
0
0
500

290045
1
0
85

290045
2
0
833

8733
0
0
66