There are two (on 4 cyl) - one before, and one after catalytic converter. You need a special socket to replace it but first I suggest you determine which one is bad by checking the code number.
Oh, those parts store casa guy that tell you that an O2 sensor is bad perhaps never mention that there are more than one. Don't get me started! With that 12 grand price tag plus labor is way, way, way too much beyond my expectation, even though Im not not really familiar about pricing in the philippines...this is absurb.