To increase the amount of XP your Units receive from battles you can apply one of four Unit XP Boosts. Go to the Active tab inside your Inventory, scroll down, then find and activate the Boost you need - Light Infantry XP Boost, Mechanized Infantry XP Boost, Armor XP Boost, or Combat Aviation XP Boost.
NOTE: Regular Experience Boosts do not affect the amount of Unit XP gained from battles.