Hello and welcome to modding union,
Angmar unit recruitment structure does not upgrade through the normal systems such as troop production or level up upgrade. Instead they can be upgrade by putting
tribute carts into the, tribute carts are spawned at
Orc Camp,
Hillmen Villages,
Barrows and
Outposts every three minutes. Once one has spawned you can select it and right click on the building you desire to upgrade, sometimes you might have to click multiple times. Once enough Tribute Carts have been put into a building it will level up. This mechanic is known as
Tribute to the Iron Crown and is explained in the palantir of each building.