Don't do it without a dolly. I've done it both ways, up a flight of stairs with a 90 or 180 degree "turn" in them, so I have a little experience with what you're talking about. You have two options; keeping it relatively assembled and using a dolly, or disassembling it (including removing the playfield) and brute forcing it. Here are my two cents:

• Easiest way is to remove all of the balls, coin box, etc from the cab. Fold the head down and strap it down so its secure. Support the back side (either with a stool or step ladder or similar) to remove the back legs. Then gently lower the back side to the ground, then tip it up so it's lying flat on its back. Remove the front legs. Now, strap it to a high quality dolly with the largest wheels you can find. "Stair sliders" are especially helpful if your stairs are carpeted. Now you and your able-bodied friend can wheel it up the stairs without too much difficulty; the person at the top pulling it up one stair at a time, the person at the bottom helping to steady it and helping with the lifting. Not too bad at all. You will find it is way easier to maneuver around tight corners with the machine folded up vertically and secured to wheels.
• If you try to just manhandle the thing up the stairs (and I've done this before, before I knew any better), you're going to accidentally put lots of stress on the legs, as you will end up having to support it at weird angles on two or even one leg as it snags on your stairs or you get stuck trying to make the turns. Be prepared to sweat profusely, and if you have kids, I recommend having your wife take them out for ice cream so they aren't exposed to all of the swearing that will ensue. They will also be protected from the 250 lbs of wreckage when you accidentally drop it down the stairs at the end.
• If you really, really don't want to use a dolly for whatever reason, do as everyone else suggests and remove the head, remove the playfield, and carry things up one at a time. You definitely won't be saving any time or effort with this route though.

And to be clear - when we're talking about dollys, we are not talking about pinball lift carts that are designed for moving pins around nice, flat workshops or game rooms. We're talking about the kind of dollies made for moving heavy shit in and out of trucks and up stairs. Get one - you won't regret it.