( informal ) used to say that if sth works well enough, it should not be changed
IF IT AIN'T BROKE, DON'T FIX IT
Meaning of IF IT AIN'T BROKE, DON'T FIX IT in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005