Search. When you make a thread asking "what needs to be fixed?" It seems that you are unwilling to put in the foot work to educate yourself and fix it right. We also can't guess what mods are or are not on the car, nor how many miles.
P0420 is simple.
-Needs a tune because of downpipe change
-Needs new o2 sensor (bank 2 aka one in downpipe post cat)
-Needs to cat in downpipe because old one is shot
These are your 3 indicatitors for that code. Start small, so if you don't have an aftermarket downpipe, then replace the o2 sensor. If you still have it after that, replace the downpipe.
Any more spoons to be filled?