After his last health scare, he decided to turn over a new leaf and quit smoking.

turn 在这里是动词，意思是"转变、改变"。 over 是副词，表示"完全地、彻底地"。 turn over a new leaf 是一个固定短语，字面意思是"翻开新的一页"，比喻"改过自新、重新开始"。