Field Upgrades are a brand new addition to the Call of Duty franchise, and are useful for multiplayer games. You can equip any Field Upgrade, and these will serve as your character's skill that you will be able to use at will when playing. Every start out by having the "Munitions Box" Field Upgrade equipped, but after you have unlocked new Field Upgrades, you can change them at any point.
In order to change your Field Upgrade, head to the weapon tab in the multiplayer menu. This will allow you to change weapons, as well as having a lot for changing Field Upgrades. Simply click on this box and look for whichever Field Upgrade you want to equip. Just click on X/A/mouse button once you find the Field Upgrade you want to use, and save your character's equipment. Now your new Field Upgrade is ready to be used the next time you join a multiplayer game.
You unlock new Field Upgrades as you level up, so keep grinding XP to get access to more and better Field Upgrade skills to use.