leave well enough alone

leave well (enough) alone
let well (enough) alone

(to avoid more action on something; to leave something alone; to avoid attempts to correct, fix, or improve what is already sufficient; to be content with things as they are and not try to improve them; not change something that is satisfactory) — воздержаться от вмешательства во что-л. из-за боязни испортить дело; не вмешиваться, чтобы не испортить; не пытаться улучшить что-л. хорошее; оставить так, как есть
...... My car works all right, so I left well enough alone and did not try to make it work better.
leave well (enough) alone (=)
.....In economic matters, they should leave well alone. (LDOCE)
Leave well enough alone AND Let well enough alone
(Do not try to change something lest you make it worse)
≅ От добра добра не ищут; ≅ Лучшее — враг хорошего;
≅ Хорошего помаленьку; ≅ Хорошего понемногу.

quit while one is ahead
“Better a little fire to warm us than a great one to burn us”

The idea behind this expression dates from ancient Greek times, specifically Aesop's fable about a fox who refused a hedgehog's offer to take out its ticks lest, by removing those that are full, other hungry ones will replace them. (answers.com)