You don’t.
Kilonewtons (just like newtons) are units of force. Pounds per square inch (psi) are units of pressure.
Isn’t this where I came in? 
Or maybe I’m just being whooshed.
You’re not being whooshed. I’m just that thick.
I’m not “getting” the difference between pressure and force. Prehaps if you could express the answer using esamples in clear visual terms so I can “see” it? I’m a fairly visual learner.
Just think of it as how its expressed. PSI stands for pounds per square inch. Pressure is defined as force (in this case, pounds) per unit area (inches). Now, you could take that kN figure, convert it to pounds-force, and then calculate what the psi would be if you were applying it with different surface areas. 20 kN will behave a lot differently if you’re applying it with something like a manhole cover (I know it’s odd–it was the first large object that came to mind) than something like a pencil. In the first case, the psi would be lower because you’re applying it with something big. You may not even damage any tissue significantly, other than some bruises. With the pencil, as you can imagine, it would poke right through the skin.
Does that help?
As mentioned, pressure is force over area. You could shove someone with the same arm force, using your hand (bigger area) or your finger (smaller area). The pressure using your finger would be bigger.
When you talk about an impact force, it is often assumed you are applying it over a small point. Obviously this is an simplified view.