How do you enhance troops for the quests?

Troops are upgraded in the command center. You need to have researched the appropriate promotion level in Vehicle Training / Fighter Training or Shooter training in the Institute first though.

There is a difference between Promoting and Enhancing your troops. Both actions need to be researched first.

To promote troops you need to research under Basic Combat. Here are items to research for each troop type and for different levels of troops.

Enhancement research is currently found under the individual Vehicle / Fighter / Shooter Training research sections. You need to work your way through a few other items first.

For the comment asking how to enhance.

Firstly is looks like low level troops can't be enhanced, this may not apply to you though.

When you go to the command center - troops page. Tap on a type or troop and you should see 2 buttons appear below. The first is Enhancement and the second is Promotion.

If you only see one then that type of troop cannot be enhanced. I don't know why.

If you tap on the Enhance button and the research hasn't been done yet it will give you an error message.

I have seen on several occasions the messages telling me I cannot promote or enhance when i should be able to. It is a glitch and restarting the game should clear it.