If you can scrape it off the best. Use protective eye wear. Repair and smooth. If too hard, you can go over it with 1/2" drywall and finish smooth. Be sure to use long enough screws to secure to joists.
Cover the floor with plastic drop cloths. Hot water in spray bottle. Wet down a small area and scrape off with a 6" putty knife. You can use a cookie sheet or shallow box to catch the scrapings. Patch any damage with drywall patching compound.
My husband and I did this when we moved in a few years ago. We filled a spray mop (one that had a soft, washable pad) with a mixture of warm water and dish soap and sprayed the ceiling enough to loosen the popcorn texture. We then "mopped" away the texture and dried the ceiling underneath with microfibre cloths. We worked in small sections, and once all of the texture was gone and everything had dried out, we added a couple coats of fresh, white ceiling paint.