How hard is it to replace oxygen sensor??

How hard it is to replace a oxygen sensor on a 1994 Merc. Grand Marquis? We had it checked on a computer diagnosis machine and that was the code that came up. Also where is it located? thanks alot.

Any store that rents tools will have the specual socket to fit them. On my '95 they are right behind the cats and easiy to get to from under the car.

I just bought two on Ebay for 65 bucks shipped, so shop around as you need to replace them in pairs.