There's no way to actually repair the old Catalytic converter so the only thing you can do is have the old catalytic converter taken off and then replaced with a new one.
Changing out a catalytic converter will require cutting off the old one and then welding a new catalytic converter in place.
The time it can take to replace a catalytic converter could be anywhere from 2 to 4 hours and can cost you $200.00 to $300.00 just for the catalytic converter and then another $150.00 to $300.00 for labor costs.