Equippable items can be powered up to increase their main stat. You can repeat this process until the item is powered up ten times and reaches +10. Each time an item is powered up its main stat is increased by a tenth of its possible roll and increased by the rarity modifier. This way the item is not only better originaly if its higher rarity, but also during its power ups.
Example: A main stat of MIN 40 and MAX 50 rolled 45. A common rarity item has a chance to increase its main stat from 4-5 on each power-up. A legendary rarity item has a chance to increase this item from 6-8.
To power up an item you need to visit the upgrade item window that can be seen by clicking on any equippable item in the character window. Each power up will cost you gold. The gold cost increases as the power level of the item goes up. After each power up, the difficulty of success increases.
The item below has been powered up 2 times. Powering up items is the core of improving stats of each Adventurer you recruit.