"If it ain't broke, don't fix it" means the thing you are doing is working well, so don't change it. Example: "You think Tiger Woods should change his golf swing? Are you kidding? If it ain't broke, don't fix it."

"If it ain't broke, don't fix it" says that what you are doing now is already working well so there is no reason to change it and do something different.

If people want to change something that is already working well, you can tell them it is not a good idea by saying: "If it ain't broke, don't fix it."

"If it ain't broke, don't fix it" means that if you are already getting the results you want then do not change what you are doing.