Unbolt the U clamp. Then you usually have to heat up the joint with a torch and pound it off. Otherwise cut the muffler off close to the case so you're not cutting the cat's output. Then banana peal the muffler pipe off the cat.
If your cat is bolted on then it will be easier to remove the whole cat and muffler together. More then likely you will need to cut the pipe off the output of the muffler to get it out anyway.