It's not going to be easy. It shouldn't be rivets, it should be screws into the top metal framing of the side panels where the roof and side panel meet Those screws rust and break behind the removable rubber piece on the molding. I replaced all of my roof screws with SS self-drilling TEK screws. To get to area of separation, first you have to lock the spring on the roller, remove the topper roller and lay it on the roof, bring in the slide to make access to the screws easier. Don't let water run between the trim piece and the roof. It will delaminate the plywood from the outside panel. I've been down that road. That is why you have to check the caulk on the top of the trim piece regularly. I'll get back to you with the SS screw size.
Gary 05 AMB DST