If you complete the movement tutorial, you'll earn this along the way, as one of the steps is to use fallback.
If you didn't earn it there, you can easily earn this at any time by sticking a ranged unit in the melee zone of an enemy (look for the crossed swords icon on the tiles when planning the move), then you will be able to trigger the fallback from your action bar (looks like a big arrow). Very easy to earn.