replacing oxogen sensors

I am fairly new to working on cars but my g/f's aspire has had me in to have to learn everything.lol...But i have to say with 122,000 its running good.But neways the gas mileage is poor and i know someone said about recplacing the oxogen sensors but i was just wondering if ne one could tell me how to do that.Every problem that this car has had i have done it myself cuz its a simple little car and i enjoy doing it so i dont really want to have to take it in.If any one could help me id appreciate it.

Thanx,
new guy
daveid

Is it the down stream, or upstream sensor. I just replaced the downstream sensor in my Aspire. It just screws into the exaust pipe right behind the Cat, then the wires just plug in. Took me like 10 min. I think the upstream sensor is the same, this one is before the cat.

these are simple but hard...? there is two one jsut after the exhaust manifold and one just before the CAT, you have to buy a special socket to do these becuase of the wire that is in the way, and they will be hard to turn if someone put anti-sieze in there, it makes it stick slightly but not sieze, and member if u got bas gas mileage its more liekly fuel filter and plugs, wires. cap and rotor, basic tuneup.