How to promote your hero to the next star?

By collecting the required materials necessary from "Promoting" the officer. Take note that each type of officer require different supplies.There are grades for each of the items too. Usually, you only get the base material for each item, and by fusing them (automatically done in the promote window), you get the required higher tier supply item. Of course, as the star ranking gets higher, more supplies are needed for you to collect. Officer Supply boxes are earned from: bug-killing event, as bonuses for individual events, as reward from Special Operation, or part of rewards in the Tier 5 Daily quest. Hope this helps!