How to enhance fighters ?

You can enhance fighters (and other troops) in the Command center. Select Troops on that building and then tap the troop set you want to enhance and tap the enhance button. (cog with arrow).

But you can only do this if you have unlocked enhancement for that troop type. Enhancement is a research item that you need to unlock. In the fighter training research tree about half way down is the item for this 'Fighter Enhancement'.