Ignition Cylinder

hi Evry1..............hopefully someone can provide me some info..............I have a 1992 Mercury Grande Marquis.............the key works to start the car................but when I try to turn the car off..........the ignition just "slips", and the car doesn't turn-off...........the key even comes right-out while the car is still running...............been told I need a new ignition cyclinder..............can any1 provide info on removing OLD ignition cylinder and installing a NEW one? Is this a DIFFICULT-TASK????
Thanks For Your Help,
Chris