Upgrading is an important, though expensive, process. Every hero has an Upgrade Level, ranging from +0 to +7. As a hero gains Upgrade Levels, their ATK, DEF, and HP increase. Upon reaching +7, and assuming they are at level 30 and below a 6-Star Grade, they qualify to be Combination Material.
About Upgrading
To access the Upgrade screen, select the hero you wish to Upgrade from the Hero/Item screen. From there, select Upgrade on the right-hand side of the screen. You'll be presented with a selection of heroes to use as material on the right. You may use up to six heroes as material at once to Upgrade your chosen hero. Depending on the chosen hero, it costs a set amount of Gold per material chosen. Also, the grade and upgrade level of the material hero dictates the amount of points the chosen hero will be upgraded. Once you've chosen your materials, just hit the Upgrade button.
Information Table
Additional Notes
It should be noted that, on occasion when Upgrading, an event may occur where the word Great or Excellent will pop up over the selected hero. When this happens, a multiplier is applied to the gained experience of the current Upgrade, x1.5 for Great and x2 for Excellent.
Also, if a hero is equipped with anything when being used as Upgrade material, the equipped items will be lost.
Finally, there exist three heroes known as Red Hatzling, Green Hatzling, and Blue Hatzling. They are 1-Star, 2-Star, and 3-Star heroes respectively and will provide triple the Upgrade experience for their grade when used as material.
Extreme Upgrade
As of a recent update, the method of Extreme Upgrading has become available as a means to surpse the +7 upgrade limit on 6-Star heroes. Extreme Upgrading works similar to a Combination, however there are different requirements.
To perform an Extreme Upgrade, the base hero need to be of grade 6-Star, be at level 30, and have an upgrade level of +7, +8, or +9. Then it must be combined with a second copy of the base hero. However, level and upgrade level of the material hero is irrelevant. This results in an upgrade level +1 higher than before.