SQLite 3.27

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

CREATE TABLE tablename(datecol TEXT)
✓
INSERT INTO tablename(datecol) VALUES
  ('1/1/2021'), 
  ('5/11/2021'), 
  ('25/1/2021'), 
  ('11/12/2021'); 
✓
SELECT * FROM tablename;

datecol

1/1/2021

5/11/2021

25/1/2021

11/12/2021

SELECT SUBSTR(datecol, -4) || '/' ||
       SUBSTR('0' || (SUBSTR(datecol, INSTR(datecol, '/') + 1) + 0), -2) || '/' ||
       SUBSTR('0' || (datecol + 0), -2) date
FROM tablename

date

2021/01/01

2021/11/05

2021/01/25

2021/12/11

UPDATE tablename
SET datecol = SUBSTR(datecol, -4) || '-' ||
              SUBSTR('0' || (SUBSTR(datecol, INSTR(datecol, '/') + 1) + 0), -2) || '-' ||
              SUBSTR('0' || (datecol + 0), -2)
✓
SELECT * FROM tablename;

datecol

2021-01-01

2021-11-05

2021-01-25

2021-12-11