As the poster above said, there are two common types. The kind with the screw (usually a Phillips screw on the end of the drum) you're just supposed to loosen the screw enough to take the "squish" out of the drum and then ideally the paper slides off. In reality, there's still a lot of friction and if the paper has ever slipped on the drum (especially while hot) it can develop rubber residue on the back that creates even more friction. In my case, I just find the spot where the sandpaper overlaps itself (the seam) and tear it or cut it with a razor knife and then peel the paper drum open like unraveling a can of buscuits. Then the new paper drum usually slides on much more easily, and you can retighten the screw.
The other kind is much easier. You just pull the head of the drum as if you're trying to pull it off the shaft and it will pop up, taking the pressure off the inside of the paper. Then just replace and push it back down on to the shaft to secure it.
Not sure how to identify the grit if you don't have the package it came in. Maybe unravel one and see if it's written on the back like most sandpaper? Not a lo of space for numbers though.
