How to remove Paint buildup where I removed chair rail on an orange peel wall
You are going to need to use a sharp tool such as a very sharp chisel or a straight blade razor scraper. I prefer a very sharp chisel as I feel it gives me better control.  It is not going to be easy, but as carefully as you can cut down and sideways from the painted area. The idea will be to cut the hump of dried paint off without too much damage to the drywall. Once you get the feel of it, you can control the "planing" cuts with your chisel. Sand down any irregularities and then use some Spackle to fill any gouges or remaining dips.  Sand for a final finish.  This method may not give perfect results. 

The second way, and most work but better results is to completely cut out the paint line with a utility knife leaving a "V" indent on the wall. After making your "V" cut out on the entire paint line,  use joint compound to refinish the damage.