Code P0135

Hi New User here. I have a '97 Jimmy that has been having some odd problems. Once in a while it will hesitate on the freeway and the SES light will come on. I had it scanned and it pulled code P0135, indicating that it is a bank 1 o2 sensor near the cat. conv. which in turn is leading to a misfire? Anyone ever had this problem? How tough is it to change?
I appreciate any feedback.
Thanks

P0135 - HO2S Heater Circuit Bank 1 Sensor 1

Driver's side manifold O2 sensor needs to be changed.