Oxygen Sensor W204

My 2013 C300 is throwing P0133 and P0420.  The P0133 is the Bank 1 Sensor 1 oxygen sensor.  Can this be accessed from the top of the engine compartment or from underneath only?  Also, do you think replacing that sensor may resolve the P0420 Catalytic Converter issue?  Still under the emission warranty so hopefully if it doesn't that will cover it.  Thank you!