Turn over a new leaf 

To change one's way of acting or thinking in order to improve oneself.

改变一个人的做事或思维方式从而得到进步

John ate so much he gained 15 pounds. This new year, he has decided to turn over a new leaf and eat healthy.

John吃得太多了，长了15磅。这个新年，他决定改过自新，健康饮食。