Take everything off you can and move it in pieces.
Nail 2x4's or a piece of plywood to the steps as runners and slide it down (assuming you have a side that is flat piece of steel). You can tape a piece of cardboard to the side of it too. If you have a place at the top of the steps either like a rafter preferably right next to the sill you can attach a eyebolt for a pulley to help control its decent. On flat ground you can roll it on pipes.
You may also want to consider adding some temporary support to the steps depending on how they were constructed. Most are not made to hold a 600lb. boiler and a few grown men.