How to upgrade or level up your units
In order to level up a unit in the game, you will simply need to have two unit of a similar star level. So leveling up a unit to 2 stars requires two additional 1 star unit, while getting a 2 star unit to 3 stars requires two additional 2-star units.
In other words, getting those 3-star units is extremely difficult and expensive, but with a bit of planning and luck, it can be achieved.
Three star units are extremely good, so always aim to get at least one of those until mid to late game rounds in order to stand a chance of winning.