Typically they are held in with some hd clips - you may need to pry them open with a flathead screwdriver. They may be bolted in, in which case you need a socket. Get a jack, even a crappy bottle jack will usually work in this application and jack up the side you are working on and change it out.
I just changed one out. Did the same thing. But it was on a columbia. Squeeze the clips and remove the bag but you'll need to have the cab jacked up bc if not the weight of the cab is on the bag and you won't get it out.