The oxygen sensor is mounted on the exhaust system and on the exterior of the vehicle. It is located after the catalytic converter and before the resonator. See pic 1 for location.
Now, because there is a wire attached to it, you can't use a standard socket for removal. You have two choices. First, try to remove it with an open-end wrench. In all honesty, they are usually rusted in place and that type of a wrench usually will round off the hex, so don't allow that to happen if it seems too tight.
Second, you need to get a socket designed for the removal and install of an O2 sensor. It will be open on the side allowing the wire to be routed out of the socket. That is the best choice of tools.
You don't need to remove the seat. Also, when looking at the attached pic, make sure to read the text in the box.
Also, here is a link that explains in general how an O2 sensor is replaced. I thought it may be helpful for you.
Let me know if this helps or if you have other questions.
Wednesday, November 18th, 2020 AT 7:07 PM