SQL Server 2017

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CREATE TABLE T(
Platform VARCHAR(25),
[Date] DATE,
ID INT
);
INSERT INTO T VALUES
('web',        '2018-10-10' ,  1),
('mob',        NULL         ,  1),
('mob',        NULL         ,  1),
('web',        '2018-10-15' ,  2),
('mob',        NULL         ,  2),
('ntl',        '2018-10-09' ,  3),
('web',        '2018-10-12' ,  3),
('web',        '2018-10-11' ,  4),
('mob',        NULL,           3);
9 rows affected

UPDATE T
SET [Date] = D
FROM
(
    SELECT ID, MAX([Date]) AS D
    FROM T
    WHERE [Date] IS NOT NULL
    GROUP BY ID
) TT INNER JOIN T ON T.ID = TT.ID
WHERE T.[Date] IS NULL;

SELECT * FROM T;


Platform
Date
ID


web
2018-10-10
1

mob
2018-10-10
1

mob
2018-10-10
1

web
2018-10-15
2

mob
2018-10-15
2

ntl
2018-10-09
3

web
2018-10-12
3

web
2018-10-11
4

mob
2018-10-12
3

Platform	Date	ID
web	2018-10-10	1
mob	2018-10-10	1
mob	2018-10-10	1
web	2018-10-15	2
mob	2018-10-15	2
ntl	2018-10-09	3
web	2018-10-12	3
web	2018-10-11	4
mob	2018-10-12	3