o2 sensor 2010

was doing a routine check yesterday with my code finder and it spit out the o2 sensor heater circuit malfunction. said bank 1 sensor 2. bought this 2010 new and have about 85k and replaced the slave cylinder and thats it. been very lucky with this car. i purchased what was called the "downstream" sensor so far and thought if there is a down stream there must be a upstream too. should replace both if there is two. most of the time there are two with one easy to get to and maybe another in the exhaust under the car. one up front looks easy to replace but what about the other? thanks.