How to solve P0137 O2 sensor bank1 sensor2

My check engine light is on, and the code is P0137: Oxygen sensor circuit low voltage (bank 1 sensor2).
Is it necessary to deal with this problem immediately? It seems this one is the downstream sensor which is just for inspection the cat. The cat has been replaced one year ago due to theft.
If I can replace this sensor myself, is there anyone know where it is? poor resource online and I didn't get useful information