Please choose the hero to which you want to add EXP, then click the "+" button below the EXP bar. After clicking the button, EXP items should appear (if you have any). After selecting an EXP item, you can tap and hold "use" to continuously use that item.
Other than leveling up your heroes by battling, you can also use HERO books. Sometimes these books can be purchased in the General Store.\n\nNote: There are two types of leveling:"Player Level" can be leveled up by collecting EXP points; "Hero Level" allows level up by collecting hero points or using HERO books. "Player level" determines the "Max Hero level" that you can level up your hero at that point in time.