1. The man promised the judge that he was finished with his life of crime and that he was ready to turn over a new leaf.
2. I know Phyllis says she’s going to turn over a new leaf, but she continues to follow the same ways and befriend the same people.

Compare to: wipe the slate clean. The expression originates from the idea of life as a book full of pages (leaves). When one turns over a new leaf, one leaves behind a spoiled page of his or her life and turns to a fresh, clean page to start anew.