P0420 is the right bank and P0430 is the left bank, so, just one catalytic converter needs to be replaced (left side, upstream). It is a bolt-on part, but the manifold heat shield is in the way and trying to remove the shield often results in broken bolts. Most will trim away the part of the heat shield that is in the way. Removing the left-front wheel well liner gives access to the top side of the converter. The other issue is that the bolts/nuts can get rusty, so, a torch may be required to get some of them off. The main muffler assembly will need to be disconnected from the left and right, downstream converters. Then, the left bank, downstream converter can be removed so that there's room to remove the upstream converter from the bottom of the vehicle. You'll likely need some new hardware and you'll definitely need some gaskets. I would also recommend getting some new 10MM push clips to replace the ones on the wheel well liner that will likely break when you try to remove them.
Another option you can do to keep the light from triggering is install a CEL eliminator on the left bank, downstream oxygen sensor. This is essentially a long spacer that restricts some of the exhaust flow to the rear sensor and "tricks" the ECM. The run anywhere from $10 to $30, depending on what brand you get. I paid $12 for mine from Amazon and the CEL for the P0430 only came on once in two months, whereas it was triggering several times a day. I keep a $25 OBD II code reader in the glove box to turn the CEL out when it happens. If you have CA state type inspection, they will fail it if they see it. It's not an issue where I live.