I've had to do this a couple of times. Here are some things that worked for me:
1. Get hold of some of the webbing straps that are used to tow light vehicles. You can buy these at Wal-Mart for a few bucks. They're usually about 8' long.
2. Get hold of some good strong rope - not the thin synthetic stuff that bites into your hands under load, but some old-fashioned 1" manila or something like that.
3. Get a heavy tarpaulin or a piece of old carpet, sufficiently long to reach down the stairs.
4. Get some work gloves - the thicker, the better!
Place the carpet as a "runner" down the stairs. (It must leave space at the edge for feet - trying to stand on a slanted carpet leads to all sorts of bad language, bad temper, and (sometimes) bad injuries!) Place the safe at the head of the stairs, with the locks/handles facing toward the stairwell (so that they'll be on top when the safe is reclined).
Position one or two of the towing straps beneath the safe, with the looped ends coming up either side (or one at the sides, and one at the front and back - whichever is easier). Thread rope loops through the loops at the end of the strap(s), and get your strong guys (wearing work gloves) to hold onto the ropes. Position one or two other guys at the bottom of the stairs (making sure they have an escape route handy in case someone lets go of a rope!
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Tilt the safe onto its back, and slide the edge over the top of the first stair. The guys on the ropes will have to take the strain at this point, but with work gloves, it shouldn't be a problem. (Hint - make sure the ropes are long enough that they can hang back behind the safe, so that someone can stand in the middle, hands on the safe, to steer it.)
Slide the safe down the stairs, slowly, steering to prevent it hitting the walls. The guys on the ropes can help with this by pulling tight on one and loosening the other, if necessary. The guys at the bottom of the stairs can also lend a hand with steering if needed.
At the bottom of the stairs, when the base hits the floor, keep tension on the ropes while the guys at the bottom bring the safe into an upright position.
Hope this helps...