How to change the vacume igniton on my mercedes 190d

[mercedes190d]

first id like to say hi to everybody,ok my problem is,I have a 1991 mercedes 190 diesel, when i go to turn my igniton off my engine continues to run.ive been told by mercedes that it is my vacume igntion switch! so i went to mercedes and brought the peice a few days ago,could anybody out there tell me how to change it step by step? any info would be much appreciated.

Cheers
John