There are two oxygen sensors on your Rendezvous. One is located in the exhaust manifold; and the other is located in the exhaust pipe behind the catalytic converter. The sensor is removed by disconnecting the electrical plug and unscrewing the sensor from the manifold or exhaust pipe. Replacing the sensor involves the same procedures, in the opposite order. The electrical plug has a latch which must be released before the two halves of the plug will separate. This may require some common sense and experimentation. The sensor body can sometimes be unscrewed with a crescent wrench; but there are also specially designed deep oxygen sensor sockets which have a slot in the side so the sensor wire can pass through without preventing the use of a ratchet drive. It will be necessary to work underneath the vehicle; so you might need a hydraulic jack and jack stands for safely supporting the vehicle. Other than that; this is a relatively simple and straightforward operation.
Here's a link to a photo of the front (manifold) sensor: (http://www.rockauto.com/catalog/moreinf
And here's a link to a photo of the rear sensor: (http://www.rockauto.com/catalog/moreinf