Oh geez, a stripped screw. I had to repair 2 small old 3DSes and getting the back off was hell, because the screws were stuck. The first one I used a drill to get it off, the second one, the screws were stuck, and it felt like I'd strip it if i continued to try and screw it off. So, I bought some WD-40, and it helped me get the screws off np.
If you mean the back cover plate is stuck on, just buy a pack of cheap pens, take the pen apart until you have only the plastic part left, get a lighter and melt the tip, and jam it into the screw. I opened my N64, Gamecube, and OG Xbox doing that, because I lacked the screw driver bit.