If you say that you are going to turn over a new leaf, you mean that you will start to behave in a better or more acceptable way.

For example:

He realized he was wrong and promised to turn over a new leaf.

She turned over a new leaf and began getting to work on time.

Every New Year’s, we make resolutions to turn over a new leaf.

I have made a mess of my life. I’ll turn over a new leaf and hope to do better.

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