How do you disassemble the alternator belt on a 1992 Jaguar XJ6 vanden plau?

That's Vanden Plas.

Buy a Haynes Repair manual for the car. That $14.00 Book will save you hundreds to thousands. I'm not giving you a fish; I'm teaching you how to fish. As a fellow Jag (V12) owner, this is the best way I can answer your question to help you now, and in the long term.

If it has a "V" belt, loosen the alternator or air pump. If it has a "Serpentine" Belt, use a breaker bar with a deep well socket and retract the tensioner/idler pulley that keeps the belt tight.