It's a bit of a pain, but you should be able to unscrew the screw on the front of the rail and push the hyfax out through a track window. Install the new one the same way. It will help if the snow is gone. And I like to use some kind of lube on the hyfax to get it to go on smoothly. Do both sides, if one side failed, the other is sure to follow.
It also helps to loosen the track tension and push the rear driveline forward to help with access. And some kind of stand to prop the back up is nice.
You could also do it by dumping the sled on it's side, which will give you good access. That works best if your fuel tank is not completely full, as you may spill if it is. I would also remove the oil cap and put a plastic bag on the opening, then screw the cap back on to keep the oil from weeping out the vent hole in the cap.
It also helps to loosen the track tension and push the rear driveline forward to help with access. And some kind of stand to prop the back up is nice.
You could also do it by dumping the sled on it's side, which will give you good access. That works best if your fuel tank is not completely full, as you may spill if it is. I would also remove the oil cap and put a plastic bag on the opening, then screw the cap back on to keep the oil from weeping out the vent hole in the cap.