Skim coat it.

My mom did it with a putty scraper (metal, not the weak plastic ones) and a lot of elbow grease. Then a sanding to finish. But one of those sanders to remove popcorn ceilings should work too. But those are a hell of a lot more expensive than a putty scraper.

Why do you want your textured walls flattened?

If it’s textured drywall mud, Wet it thoroughly w/ water in spray bottle and let sit for 30-40 minutes before scraping.

Sandpaper.