I squeezed my 180g rimmed glass tank into my fish room on end like this, the hallway leading up to it is only 4ft wide, so I simply couldn’t make the corner through the fish room door with a 6ft tank! (I’ve since upgraded to 4ft wide double doors; problem solved!)
Going down a flight of stairs will be a bit more tricky… I think of setting the tank on end (on some nice foam) by simply tilting it more once the bottom edge reaches the stairwell… then, spin it 90 degrees, still on end, and lay it back down horizontally to continue down the next flight!
Alternatively, is this a walled in stairwell? Or is one side open to air aside from a banister? If one side is open to air, I’d consider looking into a manual or electric beam/pallet lift rental!
— this would be positioned to raise up and accept the tank from the second story, so it can lower the tank down smoothly to ground floor!