You can increase the officer's level by going to Officer Hall, tapping on Military Academy, or tapping on the Officer icon on the bottom side of the main screen, then choosing the officer you want to increase his level. You will find the "Upgrade" icon below it. Click on it, and you must have the officer EXP items in your inventory.

You can earn Officer EXP items by:
- Defeating the rebel forces.
- Collect them from event rewards.
- Purchase them from some packs.