1997 Mercury Marquis o2 sensor

What "upper sensor" are you referring to? There is nothing by that name.

OK, I see now you are referring to the upstream 0/2 sensor. There are still 2 of those but they are pretty much the same. I know the plugs are very hard to reach on those. I usually use a long, thin screwdriver to release the locking tab and then pull on the wire. I use the same screwdriver to get it plugged back in later.