Can Plastic (Acetate) Frames be Adjusted?

Yes plastic frames can be adjusted to better fit you face. Maybe it's too tight, hurts you ears or does not sit right on your face. But most of these issues can be resolved by having them adjusted for you By an optician.

You can simply go to a optical near you and just tell them the problem you are having, and usually they would adjust the frames for free even if you did not buy from them, if so always try and give them atleast something for it as they did just do you a favor. Maybe give them like $5 so you can treat them to lunch, I'm sure they would like that.

Also try and not go to chains like Lens Crafters, Sunglass Hut etc... as they are more picky about adjusting frames not bought from them as they do not want to be responsible for a broken frame. But unless you guess something clueless, this rarely happens. So try and go to your local optical.