Hi Chipper, thank you for your feedback.
The upgrade system is the one which was left a little bit unexplained, we are aware of this and we are sorry if this ruined you experience.
Each hero is represented by a single dice, and you choose to upgrade the faces when you meet the dice-o-mancer.
The confirm option grey out when the cost of the faces your about to buy exceeds your actual amount of experience (represented by the blue star). The tutorial was heavy enough and we decided to left the player figure this out, maybe we should have split the tutorial in two parts and present each part just before it come into play