What does the phrasal verb "brush up on" mean in the follow...

September 7, 1991

Dear friend,

       I do not like high school. The cafeteria is called the "Nutrition Center," which is strange. There is this one girl in my advanced English class named Susan. In middle school, Susan was very fun to be around. She liked movies, and her brother Frank made her tapes of this great music that she shared with us. But over the summer she had her braces taken off, and she got a little (1) taller and prettier and grew breasts. Now, she acts a lot dumber in the hallways, especially when boys are around. And I think it's sad because Susan doesn't look as happy. To tell you the truth, she doesn't like to admit she's in the advanced English class, and she doesn't like to say "hi" to me in the hall anymore.

      When Susan was at the guidance counselor meeting about Michael, she said that Michael once told her that she was the (2) prettiest girl in the whole world, braces and all. Then, he asked her to "go with him," which was a big deal at any school. They call it "going out" in high school. And they kissed and talked about movies, and she missed him terribly because he was her best friend.

      It's funny, too, because boys and girls normally weren't best friends around my school. But Michael and Susan were. Kind of like my Aunt Helen and me. I'm sorry. "My Aunt Helen and I." That's one thing I learned this week. That and more consistent punctuation. I keep quiet most of the time, and only one kid named Sean really seemed to notice me. He waited for me after gym class and said really immature things like how he was going to give me a "swirlie," which is where someone sticks your head in the toilet and flushes to make your hair swirl around. He seemed pretty unhappy as well, and I told him so. Then, he got mad and started hitting me, and I just did the things my brother taught me to do. My brother is a very good fighter.

Book: The Perks of Being a Wallflower. Stephen Chbosky.