In the Workshop menu there is the option "Expand UNAC", there is around 16 tiers that need to be upgraded using funds or operation files.
The easiest way to do it:
Farm money in the mission 10, by selling ultimate weapons, you will get 1,5 million from 1 mission, the summary cash needed to upgrade all is about 40 milions.
Another way, if you don't want to fram money, is simply playing multiplayer, from every mission you played there will be automatically saved operation file, that you can choose to upgrade UNAC.