Once you’ve collected a few books, head over to the Heroes List page, tap a hero, and hit the Skills tab at the bottom of that page. Next, use your books to upgrade the skills, and you’ll be able to see the hero’s stats increase drastically.Once it becomes available, tap any lower-tier unit and hit the green arrow as you see in the above image. Next, drag the slider to the right to determine the number of units to be promoted, and hit "Promotion".The other defensive boost is called Troops Defens...