To upgrade a skill card you must combine two of the same cards with the same Plus (+) with a Cube-Skill in your combination window. To open your combination window open your inventory and click the "Combination" button. Place two skill cards of the same plus (+) and skill with one Cube-Skill or Ancient Cube-Skill into the combination window, click Confirm. Combining skill cards always has a chance to fail, Ancient skill cubes increase your chance of success by 30%. Examples: +1 Smite Card, +1 Smite Card, Cube-Skill will give you a +2 Smite Card if successful.