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Drill out the rivets using smallest bit possible.  Remove old plywood seats.  It looks like aluminum channel or angle on the edges of the ply.  I can't tell what is holding it on but see it coming loose in at least one place.   Once the old seats are off, maybe you can remove the edging and apply it to the new plywood seats.  If not, you can use simply aluminum angle on the front and back edges.  Doing that protects the ply edge and is more comfortable.  I would consider using stainless steel screws to reinstall the new ply seats.  Best practice would probably be aluminum pop rivets.