Upgrades are small skills of each hero. They have 3 variations: Offensive, Defensive and Utility. They are the smallest special abilites a soldier can have, and they are locked in the start. They can be unlocked and equipped by leveling up.
Each hero has 6 upgrades split in 3 columns. First column is unclocked at level 5, second column is unlocked at level 10, and the final, third column is unlocked at level 15. However, only 3 of 6 unlocked upgrades can be used at the same time. You have option to choose from 2 upgrades in each column and equip only one. You can change them, but you can't deactivate them totally.
Each class has a special upgrade order. Some classes can't use one type of upgrades. Upgrade types they can use depend on the role of the class. Engineer can use all 3 upgrade types.