After entering the game, click the [CAMP] button.
Click on the hero that you want to level up.
Click the [LEVEL UP] button to upgrade the current hero level to the highest level that can be upgraded into.
Upgrading can not only improve the attribute value of the hero, but can increase the level of the hero's initial skill when reaching a certain level as well.
Tips: Leveling up heroes require EXP, which can be obtained by clearing dungeons.