SQL Server 2019

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CREATE TABLE test
(
     [Id] [int] IDENTITY(1, 1) NOT NULL, 
     [lang] varchar(13), 
     [TotalProjectCost] [decimal](16, 2)
);

INSERT INTO test ([lang], [TotalProjectCost])
VALUES
    ('en',  CAST(341412.00 AS Decimal(16, 2))),
    ('es-mx',  CAST(342.00 AS Decimal(16, 2))),
    ('en',  CAST(341412.00 AS Decimal(16, 2))),
    ('es-mx',  CAST(34.00 AS Decimal(16, 2))),
    ('en',  CAST(341412.00 AS Decimal(16, 2))),
    ('es-mx',  CAST(34.00 AS Decimal(16, 2)));
6 rows affected

WITH cte AS (
  SELECT *, LAG([TotalProjectCost]) OVER (ORDER BY [Id]) prev_cost 
  FROM test
)
UPDATE cte 
SET [TotalProjectCost] = 15.8735 * prev_cost
WHERE [lang] = 'es-mx'
3 rows affected

SELECT * FROM test


Id
lang
TotalProjectCost


1
en
341412.00

2
es-mx
5419403.38

3
en
341412.00

4
es-mx
5419403.38

5
en
341412.00

6
es-mx
5419403.38

Id	lang	TotalProjectCost
1	en	341412.00
2	es-mx	5419403.38
3	en	341412.00
4	es-mx	5419403.38
5	en	341412.00
6	es-mx	5419403.38