sunroof megane 2000 reg?

hi guys, i have a megane coupe on a 2000. my sunroof will not tilt or open, had the glass out and it seems that something has broken on the runner on the passenger side, I've got hold of what I've been told is a sunroof cassette. (the whole thing)
i want to remove the headlining to remove the old one to fit the new one, does any one know how difficult this is. please help as a none working sunroof is driving me mad