The best way is to heat the head with a torch, knock it off with an air chisel, then switch bits and punch the rivet out with the air chisel.
But I know what it's like to not have such things handy. I've used an angle grinder to grind the heads off, I've drilled the heads off, I've cut the heads off with a hacksaw, I've cut the heads off with a sawzall and I've beat the heads off with a cold chisel and 3-lb hammer. All methods work, some just give you more exercise than others. Most of the time once you get the head off and the bracket off, you can give the remainder of the rivet a sharp whack with a hammer and knock it out, but sometimes it doesn't work so well and you have to cut the rivet again next to the frame. It works best when you've used a torch on the rivet because it becomes a little more pliable from the heat and you can drive it out easily with a whack from the ball end of a ball-peen hammer. But that's a bit of an acquired skill, lol.