Postgres 11

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CREATE TABLE mytable (
    myjson jsonb
);

INSERT INTO mytable VALUES
('[{"ch":"1","id":"12","area":"0","level":"Superficial","width":"","length":"","othern":"5","percent":"100","location":" 2nd finger base"},{"ch":"1","id":"13","area":"0","level":"Skin","width":"","length":"","othern":"1","percent":"100","location":" Abdomen "}]');

SELECT 
    jsonb_pretty(myjson) 
FROM mytable
1 rows affected



jsonb_pretty


[
    {
        "ch": "1",
        "id": "12",
        "area": "0",
        "level": "Superficial",
        "width": "",
        "length": "",
        "othern": "5",
        "percent": "100",
        "location": " 2nd finger base"
    },
    {
        "ch": "1",
        "id": "13",
        "area": "0",
        "level": "Skin",
        "width": "",
        "length": "",
        "othern": "1",
        "percent": "100",
        "location": " Abdomen "
    }
]


CREATE OR REPLACE FUNCTION json_array_update_index(_myjson jsonb, _val_to_change int, _dest_val int)
RETURNS jsonb
AS $$
DECLARE
    _json_output jsonb;
BEGIN
    UPDATE mytable
    SET myjson = s.json_array
    FROM (
        SELECT 
            jsonb_agg(
                 CASE WHEN elems ->> 'othern' = '5' THEN
                     jsonb_set(elems, '{othern}', '"7"')
                 ELSE elems  END
            ) as json_array
        FROM
            mytable,
            jsonb_array_elements(myjson) elems
    ) s
    RETURNING myjson INTO _json_output;
    
    RETURN _json_output;
END;
$$ LANGUAGE 'plpgsql';
SELECT 
    jsonb_pretty(new_json) 
FROM 
json_array_update_index(
    '[{"ch":"1","id":"12","area":"0","level":"Superficial","width":"","length":"","othern":"5","percent":"100","location":" 2nd finger base"},{"ch":"1","id":"13","area":"0","level":"Skin","width":"","length":"","othern":"1","percent":"100","location":" Abdomen "}]', 
    5, 
    7
) AS new_json


jsonb_pretty


[
    {
        "ch": "1",
        "id": "12",
        "area": "0",
        "level": "Superficial",
        "width": "",
        "length": "",
        "othern": "7",
        "percent": "100",
        "location": " 2nd finger base"
    },
    {
        "ch": "1",
        "id": "13",
        "area": "0",
        "level": "Skin",
        "width": "",
        "length": "",
        "othern": "1",
        "percent": "100",
        "location": " Abdomen "
    }
]


SELECT * FROM mytable


myjson


[{"ch": "1", "id": "12", "area": "0", "level": "Superficial", "width": "", "length": "", "othern": "7", "percent": "100", "location": " 2nd finger base"}, {"ch": "1", "id": "13", "area": "0", "level": "Skin", "width": "", "length": "", "othern": "1", "percent": "100", "location": " Abdomen "}]