the mast will have to be removed and laid down on the floor.
then the mast chains taken loose and cylinders removed
then you should be able to move the channels up and down to gain clearance on either end to expose the rollers.
There might be stop blocks on the top or bottom that might restrict the mast from moving the way you want it to. If it is a cascade mast then those stop blocks are usually removable.
Once you get these items out of the way the channels should move so you can get access to the rollers.
I do want to make it VERY CLEAR, be very careful when dealing with these and always mind your hands while manipulating the channels, use pry bars if needed. This kind of work is tedious and sort of dangerous. You get your hands or fingers in the wrong place while moving the channels you could pinch and hurt yourself.