I've changed the rotors on my W124 and there is just the one screw but most likely your rotors are rusted on like mine were. Spray with WD40, etc., use eye AND ear protection, start with a rubber mallet and then a BFH (big f... hammer) if neccessary. Have new rotors on hand as you will probably damage the old ones. A thin layer of antisieze paste (you'll see where to put it from the old rotor) will help with future changes.

glenmore
1991 300CE
1990 LS400