Unit Attribute Upgrade

Go To Kingdom -> Tap any unit - > you will see atributes with [+] button on it.

As it for me, its better to leave it for the late game. The price of an upgrade is alwys in Green Crystals (GC), and with each star on the unit - the price is increased. But if you want to save atributes you've upgraded on a 6* unit trying to Evolve it, you have to invest same amount of upgrade in your second 6* unit for Evolving. So you need to pay twice, while the price of one upgrade with each star is increased not That much.