When a soldier's unit interface shows an upward arrow, he can be promoted. Click on the soldier to see his development potential, and you can choose different directions to upgrade the soldier (such as shooter, spear soldier or rider). Open the arms tree, click the soldier's name with the right mouse button to see the promotion direction.

When a soldier's experience reaches a certain level, you can upgrade his rank. After the upgrade, his salary will increase, but at the same time, his fighting capacity will be stronger and his equipment will be better.