In Harper Lee's To Kill a Mockingbird, one thing Scout changes about herself over the course of the first 18 chapters of the book concerns gaining more control of her temper.
At the start of the book, Scout is very quick to lose her temper and get into fist fights any time she thinks she or her family has been insulted. However, by Chapter 9, Atticus tells her she needs to stop fighting and start learning how to use her head by remaining calm in times of difficulty. He further warns her that she'll be subjected to ridicule frequently due to his involvement in Tom Robinson's case, and learning to use her head will now be even more important than ever. At first Scout struggles to keep her head, but by Chapter 11, Scout is able to keep her head, whereas Jem loses control.
In Chapter 11, Scout and Jem begin bravely and frequently walking past Mrs. Dubose's house, the meanest old lady in Maycomb. Each time they walk past, she insults the children. One time she stoops so low as to say, "Your father's no better than the niggers and trash he works for!" (Ch. 11). Passing her house again on their way back home, Jem grabs the baton he just bought for Scout and uses it to whack off every camellia flower in her garden. Scout notes the following about her brother's behavior in comparison to her own:
What Jem did was something I'd do as a matter of course had I not been under Atticus's interdict, which I assumed included not fighting with horrible old ladies. (Ch. 11)
She further notes her surprise at his behavior since he usually has a very calm temper that significantly contrasts with her own very hot temper. The above passage shows us that, under Atticus's guidance, she has begun to learn how to keep her head, whereas Jem is beginning to be worn down by all of the ridicule he is experiencing.