Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.
You can get this at any point after mission 1 depending on what you want to upgrade of course.
As long as you have enough DNA (by killing enemies mashing the
button, collecting furon probes, finishing missions etc) you can get on your mothership, go to upgrades and pick any one to unlock this.
Go to 35:40 in my part 1 video to see where and how to unlock this
Hope this helps!