This article includes a 1:56 video walkthrough.
Quick answer
The lower back pad is held on by compression, not bolted through. With the Allen tool, loosen (don't remove) the two screws underneath, and the pad slides out of its slots. To install, slide it into the slots, then press it firmly together while you tighten each screw so there's no gap.
Step by step
Find the two screws underneath the lower back. Flipping the chair over can make them easier to see.
Loosen each screw a few turns. You don't need to take them out.
Slide the lower back pad out of its slots.
To install, loosen the screws a little more for clearance if needed, then slide the new pad into both slots and wiggle it together.
Hold the parts together with gentle pressure while you tighten each screw until snug, so you don't leave a gap.
Also called: pelvis support pad, lower back cushion, replacing the lower back, lower backrest.
Video transcript
Hi, guys. This is how you take your lower back off. You only need the Allen tool, and you're going to be loosening two bolts that are underneath these two parts right here. Underneath here are the two screws. Now, you don't have to take them out. This is just a compression fit. It's not bolted together. It's compression.
I'm going to go on this side so I'm not in the camera guy's face. I'm sticking it in here, and I just turn it a few turns. Look at that, it already came out. One's already loose. Come over to this side. Same thing. Get underneath here. Maybe you want to flip your chair upside down so you can see it. I just loosen these a little, and that's how they come out, and it just slides.
Now come around here and look at this. You can see there's a slot, and you can see that there's a screw here. They're loose, but if you're going to be putting it on and off or replacing it, maybe pull these out just a little bit to make it a little easier to slide in. And it's as simple as getting this into here, this into here, and sliding them. Wiggle it. Get it together.
And now you need to tighten it up, but you've got to make sure you put a little pressure on this. Otherwise, you might be tightening it right here and leaving a gap. So I usually go down here, get my Allen tool on it, trust my other hand, and just hold it together with a little pressure. You just turn this a few times until it gets nice and snug. Good enough. Look at that. It won't go anywhere. Come around this side and do the same thing. Don't want to leave it like this. Pull it in tight. Snug it up. There you go. Now it's on.