If the W140 is similiar/identical to the W124's its very simple/straightforward. Remove the retaining 2 pins that hold the pads in place. Remove the rear brake sensor if so equipped. Unscrew the brake reservoir cap. Using a C-clamp( or a wide flat screwdriver in a pinch) Gentle and carefully press the pads away from the rotor disc. Be sure to press them completely out as far as they will go. You will need the entire space to put the new pads in. If upon installation, the new pads don't fit, put one of the old pads back in and pry the piston back some more. If you have lip at the outter edge of the disc that's any more than a nickel in width, you may want to consider replacing them. Generously apply WD40 to the center hub area. There are 2 (17 or 19MM)bolts that hold the Caliper to the carrier. Remove the 2 bolts and the caliper will come off. Support it out of the way such that it does not dangle by the brake line. There is a 5 or 7MM allen that hold the rotor disc to the axle carrier. Remove it. Use a brass hammer or a mallet and a block of wood or gentle tap the disc directly IF YOU ARE SURE THAT YOU ARTE GOING TO DISCARD THEM until the rust is broke and the disc falls free.
Also check out this pictorial. Its for the E class, but should be of some help.