leave/let well alone
(British, American & Australian) also leave/let well enough alone (American) to leave something the way it is, because trying to improve it might make it worse In cases of back trouble, it's difficult to know whether to operate or leave well alone. I'm not doing any more on that painting - it's time to let well enough alone.
leave well alone
(British & Australian) also leave well enough alone (American) to not change or try to improve something that is not causing any problems So long as the machine still does what you want it to, my advice is to leave well alone. Surgeons are aware that every operation carries some risk, and sometimes decide to leave well enough alone.