Look at the first scene you wrote with them, then think about how you can show how they are different at the very end! How do I write this character so that you can clearly see how they changed by just reading the beginning and end of my book?
Then consider what kind of things someone could see or do that might change them! This is where you can get really creative! Feel free to use something from your own life. How have you or your friends changed over the years and what caused it? How can you magnify that on a more dramatic scale for your writing? Or what’s something that you could see changing your character in the direction you want that fits in the bounds of your story?
Don’t be afraid to break down your favorite character arcs from books and movies!! This is one of the most valuable exercises I’ve done in working on my graphic novel! Really take a moment to critically examine how another author or artist changed their characters can show how you can use mechanisms in writing to show change!