Unplug and unscrew them and plug in the replacements.
The Mustang guys say NTK is better than Bosch for replacements.
I think it's a 7/8" open end you need -- a good one with tight tolerances. They can be tough to get out. There's a special socket for that with a slit in the side that lets you get it over the cable and slide it down but I've never used one.
There's one sensor in each leg of the Y-pipe just below the exhaust manifolds. You can reach it through the front wheelwells with the wheel soff and the protector piece son the bottom off. That accounts for two.
The third one is not critical to your fuel ratio and can be left alone if it's working okay -- or you can replace it -- up to you. It's between the two big CAT's on the single exhaust pipe after the Y-pipe (on a 2002, on 2000 and previous it's after the two big CAT's).
The last o2 sensor just monitors catalytic efficiency with a broad range of acceptable values and that's why it's not critical. It doesn't correct your fuel ratio, it's just a catalyst monitor.