I've got a perfect roof in a parts car and a mostly torn one in the good car. The strips along the sides have teeth. You'll have to take a wooden or plastic putty knife like wedge and pry the retaining strips out from the ceiling.
Installation involves hanging the headliner from the metal bows.
You then take a flat piece of wood and mallet to return the retaining strip back to the original location. You don't want to have the wooden block to have the blows sharply across the edge of the retainer or it may cut the liner. I have the Volvo shop manual that concentrates on the 122S but has some stuff on the 544. I can look it up and provide the factory method if you'd like. Contact me at my email address if you'd like that.