1. Download a knockback system.
2. Install the knockback system into your map.
3. Make the configuration of your knockback into your spell.
4. Test the map.
There might be one problem.
Do you know exacly when you want it to be knocked back?
If it is a ranged unit, you want to do it when he fires a bullet.
You can do it when the bullet hits it's target but then you have a glitch where a bullet can be intercepted and you might have a small delay.
Most knockback systems will not be happy with applying knockback on attack order.
Do you have a custom event when the unit fires his bullet?