SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE Files (Columns nvarchar(max))
INSERT Files VALUES ('[{"header":"C"},{"header":"D"},{"header":"E"}]');
1 rows affected

UPDATE Files
SET Columns = (
    SELECT test = ''
    FROM OPENJSON(Columns)
      WITH (
        header nvarchar(10)
      ) j
    FOR JSON PATH
);
1 rows affected

SELECT * FROM Files


Columns


[{"test":""},{"test":""},{"test":""}]