Such an event is extremely rare and whatever plan needs to be implemented would need to be quite bespoke to the outcome.
Let's suppose that we end up calculating with >99% chance that this thing will hit Earth and that we cannot deflect it. First thing to do would be to determine where on Earth it is going to hit. Whatever plan we create would depend on that. If we see that it's going to hit in the middle of an ocean far from land, we probably do nothing other than evacuate coasts nearby in case of any tsunami. If it hits remote land (middle of Sahara desert, Middle of Amazon forest), we will have to permanently resettle some people, but it won't be a huge lift. Once we get into the possibility of it hitting a major metropolitan area, that's when we have to make big plans, and the plans would change depending on the country. That would also factor into whether or not we would want to launch a deflection mission.