I’ve used this method and it worked great: https://homeguides.sfgate.com/rid-textured-paint-wall-20277.html
You can try getting it damp and scraping it, but if it's been painted, your best bet is probably to skim coat it to achieve a smooth surface. Here is a post I wrote about removing popcorn ceiling, but that wasn't painted yet. The ones that were painted needed to be skimcoated.
Hi! Here's a method I'm planning on using for the house we are listing for sale, soon: