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To improve, refresh one's knowledge of something
提高，重温某人在某方面的知识;温习
I must brush up on my French before going to Paris next month.
下个月去巴黎之前，我必须温习一下法语。
He hasn't studied English for so long that he'll have to brush up on it. 他很久没有学英语了，所以他不得不重新学习提高一下。