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Brush up on sth
means to improve or refresh your knowledge or skill in something that you haven't practiced in a while. Once again, it means to refresh or improve your skills or knowledge in a specific area.

Examples:
1- He said he’d brush up on his dancing before the wedding, but when we saw him on the dance floor, we knew he didn’t.
2- After not driving for a few years, John needed to brush up on the rules of the road before feeling confident behind the wheel again.