Walker would give you instructions. I think they are cold riveted in place. The important part is the fit in the holes, I think they are lightly pressed in, not slide fit.

The studs have a shoulder, so the rivet part is thinner than the stud itself. The studs are quite weak and soft so when I got the new studs in hand, I would try to wiggle and break off the studs and use a dowel to pound out the remaining part from the inside. If that doesn't work I'd go for the more conventional drilling and grinding, but that can damage the seat and hole.