Welcome to CEG!
Which engine do you have? How many miles?
The "upstream" sensor is the one on the exhaust manifold. This sensor measures oxygen in the exhaust gas for the purpose of adjusting the air/fuel ratio.
If the sensor is getting old (over 70,000 miles) you may just want to replace it. There are other things that may trigger this code, but they will be difficult to track down.
This sensor is slightly larger than a spark plug, and has wires and an electrical connector on it. It screws/threads directly into the manifold. Autozone can supply the part and the tool/socket for replacement. You will need to have or borrow a 3/8 inch wratchet to use the tool.
If your engine is the I4, you will find it between the engine and radiator. If it's a V6, the sensor will be between the engine and firewall. The V6 will be much more difficult to do. Search these forums for tips.
