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Turn over a new leaf
means to make a fresh start or to change your behavior for the better. It’s about leaving old habits behind and embracing positive changes.

Examples:
1- After eating too much junk food last year, Sarah decided to turn over a new leaf in the New Year. She joined a gym and started meal prepping every week.
2- After World War II, many nations worked together to turn over a new leaf and promote peace and cooperation.