Does muscle turn into fat?

Not literally: If a person lays off a fitness regime but continues to eat as that person normally would, the atrophy of muscle from being used less and the gain in fat from eating more than is now needed leads to the mistaken idea that one tissue becomes another. Very muscular intense individuals who lay off tend to be the worst overeaters, hence many ex-bodybuilders get obese.

No. Common myth.: While exercise will help you lose fat and gain muscle simultaneously, it doesn't literally turn fat into muscle. By the same token, with a sedentary lifestyle you'd lose muscle and gain fat, but not turn muscle into fat.