Okay first, Ant695 & xRoGuE GriMMx are both factual with their information on where to get this achievement and when.
I'd just like to add another chance to get it for those who don't read the achievement list and or solutions before getting to it. I had been looking for something when I noticed these other two guides. By then however, I'd already passed the chance up being in the middle of chapter 6. I didn't know if I'd have to wait for a second playthrough.
Luckily you don't have to wait.
In chapter 6 you meet up with someone(won't spoil it). The two of you work together, and you end up going after 3 key cards. When it's time to go after the 3rd one be aware this is your chance to get the upgrade if you missed it in chapter 5.
After crossing a wooden plank bridge with two grey barrels on it you go through the cave. At the ladder turn around and go up beyond the bush on the right(if facing opposite ladder). This is before going down the ladder.
At this point ..... it's as the prvious two guides have stated. I was just laying it out there that it was okay to miss it in chapter 5.
Hope this helps someone.