how do I undo the casing of my servis M6500W?

Carol. Have you checked the outlet pipes, including the machine's flexi-pipe and the Waste pipe which the flexi-pipe goes into? This is the most common reason for flooding, so checking these and cleaning thoroughly, before any major machine dismantling would be a wise start.
If you have cleaned these or they are not blocked, is the water coming from the door seal area or underneath?
Dry the front, Puff some talc at the bottom of the door, on the bodywork and try a cycle. If the water is from the door seal it will be clearly seen by the talc being washed away. If talc gone, door seal is the problem.
If from underneath, Tilt the machine back on it's back feet. Prop it up or get someone to hold it whilst you check underneath for any split or disconnected pipework.
If the problem is the door seal, open the door, look for a clip, which hold two ends of a wire retainer on the outside of the seal. Remove this wire retainer, then remove the lid of the UN-PLUGGED machine, and at the front of the machine around the area of the drum you will note another larger retainer strip held in place and tensioned, by a bolt clip. Loosen the bolt with a spanner or socket, just enough to release the seal from the drum. PAY SPECIAL ATTENTION TO HOW THE SEAL IS ATTACHED AS YOU REMOVE IT THROUGH THE DOOR. This will avoid confusion when refitting a new one. Take the old seal to a domestic repair shop who will hopefully have the relevent seal and replace.
Good luck.

Davy Mac
June 2009