Brush up on is a common phrase which means to refresh one's knowledge or skills on a particular topic or subject. But there are other synonyms that can be used to convey the same meaning in different contexts. For instance, "reacquaint oneself with" is a synonym that suggests revisiting a topic or skill that one has not used in a while. "Polish up" is another synonym which denotes refining or improving one's existing knowledge or skills. "Review" or "revise" are commonly used terms that suggest a more thorough and detailed examination of a topic or subject. Whatever phrase you choose, knowing the right synonym can help you express your thoughts more effectively and accurately.