First off, large life-ending asteroid impacts are very rare as there aren't many of them out there and we've found almost all of them:
Looking at the 'continent' and 'global catastrophe' areas of shading on the right, the percentage discovered (blue line and numbers on the right hand vertical scale) is 80+%. This may not sound good, but if you look at the 'number of objects' (red line and numbers on the left vertical scale), you can see that is a actually only a handful of objects. (These objects are likely on odd, long period eccentric orbits which means they spend too much time being too faint to get picked up by the surveys but will eventually be discovered. If you look at the 1+km survey stats at the Center for NEO Studies (CNEOS), you can see we're find about 5 or so of these object per year now)
The bigger risk is from the 50-400 meter sized objects (the 'city' and 'region impact devastation' areas) simply because there are so many more of them (the estimated numbers are on a logarithmic scale) and we have found far fewer. The plan for finding and dealing with these objects (in the US at least) is set out in the 'National Near-Earth Object Preparedness Strategy and Action Plan' which was released by an interagency (NASA, FEMA and others) group in June 2018.
Internationally, this falls under the UN Office of Outer Space Affairs Committee on Peaceful Uses of Outer Space as outlined starting on page 7 of this NASA PDCO presentation that was given at the Jan 2018 Small Bodies Assessment Group meeting. The NASA PDCO is the Planetary Defense Coordination Office which was setup in 2016 following the NASA Office of Inspector General's report to better detect and track Potentially Hazardous Objects (PHOs), those objects larger than 30-50 meters that can come within 5 million miles of the Earth, issue warnings and study strategies and technology to mitigate PHO impacts, and coordinate US government planning for response to actual impact threat (from the PDCO overview).
Under this umbrella are SMPAG (discussed later) and the International Asteroid Warning Network (IAWN.net) which consists of space agencies and other observatories who do the tracking, modelling and would produce an official alert if the thresholds (page 8 of the presentation) were met:
IAWN shall warn of predicted impacts with greater than 1% probability for all objects greater than 10 meters in size
Larger objects (greater than 20 meters, with more than 10% probability of impact in the next 20 years) will trigger Earth-based planning which for a potential US impact would be handled by Planetary Impact Emergency Response Working Group (PIERWG), a joint NASA/FEMA group. Even larger potentially impacting objects become the job of Space Missions Planning Advisory Group (SMPAG) (pronounced "samepage") who would look at mitigation options. SMPAG is working on 11 tasks, which are covered in this overview presentation, some of which including the 'Planetary Defense Action Plan', which covers what we would do to learn more about the object and options to deflect it, are available in their working draft form from the 'Documents and Presentations' section of the SMPAG website.
Edit: As I noted in a comment to this question,
a tabletop exercise is run at each of the biennial Planetary Defense Conferences involving the conference participants; past conferences links are here. The PDC exercises are created by the Center for NEO Studies at JPL (most recent one here from 2019's PDC) are plausible but usually contrived in certain ones. The orbit is usually such that the planetary radars cannot observe it as they nail down the size, orbit and potential impact point very precisely, which removes the "making decisions based on uncertain knowledge" part of the exercise. Also as noted in the comments, an impact over a city (New York in the PDC19 example) is extremely unlikely given the percentage of Earth's surface covered by cities but it's a long-running "joke" of the PDC exercises that the hypothetical asteroid is always aiming towards the city where the conference is being held...
NASA and FEMA run joint tabletop exercises roughly every year, reports of previous exercises are at the bottom of the PDCO Supporting Documents page.