Where are the O2 sensors located on a 1998 Ford expedition and how do you change them?

They are screwed into the exaust system at various points. Some people say that there are 4 oxygen sensors, but technically, that is not correct. Only the front 2 are considered oxygen sensors. They are screwed into the exaust pipe, one on each bank, after the exaust manifold, but before the catalytic converter. Sometimes they are screwed directly into the exaust manfold itself. The other 2 are actually called catalyst moniters. They are oxygen sensors, but they serve a different purpose. They moniter the activities of the catalytic converters, thus the name, catalyst moniters. They are screwed into the exaust one on each bank,after, or downstream, of the catalytic converters. A 7/8ths inch wrench is the size needed to turn the O2 sensors. They just unscrew out normally. The connectors on the front O2's will be a challenge. They are way up behind the engine in a really stupid place to get access and unplug/plug in.