O2 sensor replacement

I am wondering when I get the PO171 and PO174 )2 sensor code. How do you know which of the four O2 sensors need to be changed. Do you start with the upper ones?

I am wondering when I get the PO171 and PO174 )2 sensor code. How do you know which of the four O2 sensors need to be changed. Do you start with the upper ones?
The codes indicate your running lean. I would check fuel pressure, air leaks gaskets at manifold exhaust leaks. Do this before working on sensors.

I believe this all started with a badly cracked intake air tube. I replace it and the O2 sensors bank 1 and 2 sensors 1 and the codes cleared. I had no air leaks there has been no loss of coolant. I repaired it with the help of code connect on my new uscan reader. Thanks for the input.