Aha tak takto ☺️ You start out by spending the Seed of Light to unlock the first of the four upgrades, but beware--the game makes it clear the seeds are "very rare," so you should be sure to pick the subclass you want to focus on first. In time, you'll get them all, but the others won't come until after you finish Forsaken's campaign and can successfully make it through the Blind Well activity, where Seeds are random drops.