1.The president of that company finally came clean about stealing seven million dollars.

那家公司董事长最终承认他挪用了公司七百万美元。

I thought it was time to come clean about what I'd been doing.

我认为是时候坦白我一直在干些什么。

come clean:

(idiomatic) To confess; admit the truth.

Synonyms:

fess up, own up

2.The police encouraged the criminal to level with them about the crime.

警察鼓励罪犯坦白罪行。

level:

(usually with "with") To speak honestly and openly with.

3.Apparently, he's turned over a new leaf and he's not smoking any more.

很明显，他已经改过自新，不再吸烟了。

turn over a new leaf:

(idiomatic) To engage in self-improvement; to begin a good habit or shed a bad habit.

Antonyms:

(determination to do what is wrong or destructive): break bad