Some cars have their catalytic converter fixed permanently to the car but in most cases you will just need to take the O2 sensor off the car and then remove the bolts located on the catalytic converter. Once done you will be able to remove the converter from the pipe it is connected to. It is advised to see a professional mechanic to help you out with converters replacement. To install a new converter it is advised to lube the pipes to connect the converter then align with the bolt and tighten them properly. Check for leaks while testing it and if it does leak then wait for the catalytic converter to cool off before tightening the bolts.