Upgrading Prowess or Stars of Hero

How can I upgrade the Prowess or Stars of one of my Heroes?

For this you need two things, the required EXP level (top bar when you select a Hero in the Hero Roster) and the required number of hero fragments (bottom bar). For example to increase to two Stars you need EXP level 10 and 40 Hero fragments. The EXP you get for killing monsters in addition to having a chance at EXP when summoning heroes. The fragments you can get when you summon heroes, for example when you already have the hero that you get, in that case you get some fragments instead.