If you mean the ignition timing - don't. You'll need a strobe light, a Haynes manual and some mechanical aptitude, but to be honest if it's got to the stage where you need to adjust it it's probably time to go to the garage. 

 

If you mean the actual dashboard clock then I can understand your confusion - Merc clocks of a certain vintage are notoriously hard to fathom. Basically you need to use the lower left button on the steering wheel until you see "Setting can be changed etc..." on the display then press the up arrow. The display should read something like "Vehicle, Lighting, Instruments" etc... use the + / - keys on the right to scroll through to "set time" and confirm. That should do the trick.