Ron, that is a push nut. You have to work it off of that unthreaded stud. If you take a small screwdriver you can peel back those retaining teeth just a little to relieve the tension and slide it right off, or you can wedge a couple of larger screwdrivers behind it and force it off. If you do it the first way you can flatten it back out and reuse it - if you do it the second way you might be making a trip to the hardware store...
Once the push nut is off, slide the seat back outward until the lower pivot bracket is off of the unthreaded stud, then tilt upward. The whole thing will come right off.
Here is what the frame looks like with the upholstery and padding removed. To the upper left you can see the unthreaded stud that the outer bracket for the seat back pivots on, and to the upper right of the picture you can see the center studs. Notice how they are angled upward. This is why you have to lift the seat back upward to get it off.
Hope this helps.
