DECLARE @t TABLE (c_id int, c_json VARCHAR(MAX))
INSERT INTO @t
SELECT 5, '{"FIO": "Mike", "age": "30"}'

SELECT c_id
     , c_json = (
        SELECT FIO = JSON_VALUE(c_json, '$.FIO')
             , age = JSON_VALUE(c_json, '$.age')
        FOR JSON PATH
     )
FROM @t
FOR JSON PATH

DECLARE @t TABLE (c_id int, c_json VARCHAR(MAX))
INSERT INTO @t
VALUES (5, '{"FIO": "Mike", "age": "30"}')
     , (6, '{"FIO": "Dick", "age": "32"}')

SELECT REPLACE((SELECT c_id, c_json = 'xxx' FOR JSON PATH), '"xxx"', c_json)
FROM @t