SQLite 3.39

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CREATE TABLE tablename (date TEXT);
✓
INSERT INTO tablename VALUES
  ('5/24/2023'), ('12/5/2023'), ('1/1/2023'), ('12/31/2023');
✓
SELECT * FROM tablename;

date

5/24/2023

12/5/2023

1/1/2023

12/31/2023

UPDATE tablename
SET date = SUBSTR(date, -4) || '-' ||
           printf('%02d', date + 0) || '-' ||
           printf('%02d', SUBSTR(date, INSTR(date, '/') + 1) + 0);
✓
SELECT * FROM tablename;

date

2023-05-24

2023-12-05

2023-01-01

2023-12-31