How to change plugs on 1997 expedition v8?

For the 4.6L it's a fairly involved job and I wouldn't recomend it unless you are getting close to 100K miles or are having misfire problems. For the 4.6L it's a matter of removing or loosening the items that are in the way (such as the air cleaner drum and hose leading to the throttle body, as well as the electrical interface plugs and relays mounted on the firewall in the engine compartment on the passenger side of the car). Make sure you cover any ports or hoses that you disconnect to prevent dust or debris from getting blown into them. Removing the spark plug boots is usually the toughest part of the job, because there isn't enough room to get your hands in there to pull the boots off. I discovered with mine that a helpful technique is to use a length of strap type rope or cord and loop it around the spark plug wire. Then tie a slip knot in the rope and manuver the loop so it is around the head of the spark plug boot. Gently pull the loop tighter so it closes around the boot (NOT THE WIRE!). Once it's tight you can use the rope to pull upwards to disconnect the boot. Remember that the plugs are slightly angled on the Expedition's heads (towards the rear of the vehicle on the passenger side and the front on the driver's side), so you'll have to pull at a slight angle in order to release the plug boots.

After that it's all very straightforward stuff (blow out the area with compressed air, use a spark plug socket to remove plugs, gap new platnium plugs and use anti-seize compound on the threads of the new ones, use silicone dielectric grease on the boots, torque new plugs to spec, etc.). If you're unsure of how to proceed get a manual from Haynes for your vehicle.