To upgrade your Units to higher Ranks, you must accumulate the necessary amount of Unit XP and construct Advanced Training Center Buildings. The Unit Upgrade tab inside each of these four Buildings (the Light Infantry ATC, Mech. Infantry ATC, Armor ATC, and Combat Aviation ATC) outlines any additional requirements you need to meet. Some of these requirements may include:
- Upgrading certain Buildings to the required Level
- Upgrading certain Tech to the required Level
- Accumulating a certain number of Medals
Medals can be earned by reaching new Levels or you can purchase them from the Store by going to the Resources Tab, opening the Misc. subtab, and visiting the Medals Section.
Note: Upgrading Units to higher Ranks is a permanent action that applies not to individual Units, but the entire class - if you upgrade your Grenadiers to Rank 3, all future Grenadiers you hire will automatically have Rank 3.