Update Trigger in MYSQL

Here is my requirement :-

I have a table called "users" and field called "Admin". Field "id" is primary key. Wants to update "Admin" field to "1" if id=1000;

Tried this ,but not working.I tried "New" instead of "old".Also I tried "After" instead of "Before". But it is not working..

Any suggestions please...

DELIMITER $$
CREATE TRIGGER update_user_admin
Before UPDATE ON users
FOR EACH ROW
BEGIN

  UPDATE users
    SET admin = 0
  WHERE old.id = 672;

END;
$$
DELIMITER ;