how do you return a canon camera that you orderd on american express?

Your camera should come with a serial number and a registration packet/info of some sort. Go to the Canon website and type in the model of your camera and it will bring you to a support page for that product. I’d suggest registering it first (as if you have to send it in for repairs, I’m sure they will ask you to later) and then going to Tech Support first to see if your problem can be solved.
If the result of your camera being broken was Not Your Fault, then continue on to Service & Repair to have your camera sent in to be fixed.