It's always easiest to remove the entire seat to work on it, rather than try and get at everything while it's still in the van. Once you get the seat out, it's a fairly simple procedure.
You need to remove the seat cover from the back rest portion of the seat. There are some retaining clips, and possibly some hog rings holding the cover on depending on the model. Once you get the cover lifted up far enough you can get at the small nuts that hold the side airbag to the seat frame.
Remove the airbag and wiring harness as one assembly.
GM Dealer Technician For 18+ Years In the automotive industry for 20+