Nissan Altima Catalytic Converter Plugged

I Took My Car To My Local Dealer For An Unrelated Problem And The Service Advisor Told Me That My "check Engine Light" Was On, He Checked It...

And The Bolts Are Rusted On. What Should I Do To Change It ? I Have A Whole Muffler With The Flex Pipe. Is It Possible To Change It ?

The Other Day My Ses Light Came On So I Went To The Nissan Dealership And Pulled A P0420 Code. I Also Notice Clunking/knocking Noise Within...

My Check Engine Light Came On One Day So I Took It To My Mechanic He Hooked It Up To His Computer And It Said That The Catalytic Converter...