Popcorn ceilings or acoustic ceilings as they are called are paint-on ceilings used in the old 1960s in America. They are considerably cheaper and can camouflage the defects (like stains, uneven geometry, etc) in the true ceilings.

As the old paints fade from your popcorn ceilings, it becomes necessary to pull out that ladder from the attic & repaint them. First & foremost, check whether your ceiling contains asbestos. To check it, you can scrape some material from a corner & send it over to a testing service. If it contains over 1% asbestos, it is mandatory to hire professionals to remove the coating.
If it doesn’t, then you are free to DIY. Cover your walls with plastic sheets to protect them. Skim & sand the ceiling to give it a smooth texture. Do not forget to use a primer. Then you can paint on 2-3 coats of latex paint followed by a sufficient drying time.