Where is the upstream oxygen sensor located on a 1999 Ford Taurus SE with a 3.0L V6 engine and how do you go about changing one if only the upstream is bad do i need to change the downstream as well?

The upstream 02 sensor is usually located shortly after the exhaust manifold, before the converter. That is the one that controls how the car runs. The downstream sensor, after the converter tells the computer if the converter is doing it's job or not. It does not affect performance, but you might have to live with a check engine light being on. There are probably 2 of each on a V6.