Looking to replace an O2 sensor. needing help.

I am new to these forums and I am trying to figure out how to navigate them (on my phone...no pc), so forgive me if this has already been addressed. I feel somewhat confident in my abilities working on my cars as I do most routine basic maintanance. But money is tight, and I need to replace (I think) an O2 sensor. The engine light came on so I took it to advanced auto and their machine said, "P2243 oxygen sensor ...". I have watched a couple vids on how to do it, which it does not seem difficult, however it was not on a mazda5. I just want to be sure before i move forward and try to tackle this myself.

Are there any tutorials somewhere?
Is this the upstream or downstream sensor code?
Best place to buy a new sensor (or any other parts)?

Fyi it is an 08' mazda 5 sport automatic.

Thanks in advanced!

Upstream sensor. Rockauto.com has a Denso brand (probably a good bet) for $91. Without shopping around further, I don't know if this is a good price, but they're usually pretty competitive. Don't forget to use anti-seize but keep the sensor tip clean.

Thank you carl!

I had to replace my upstream one at around 70k. The best advice I have heard is to overtighten it by about 1/8 of a turn to crack the seal, then unscrew it