I stripped the screw on the backplate of my old 3ds XL, wont come off

You need to create a new slot on your screw head. You can use a Dremel or any small rotating tool with a cutting disc, cut a simple slot then unscrew the problem screw with a flat screwdriver. If you have any doubts about how to do it, there are videos on YT explaining this.

Spacec4t recommended,and gave good advice.

You can also use super glue. Put a tiny drop on the screw(be careful not to be messy). Then attach an Allen wrench or something small and preferably flat(and something you don’t mind getting glue on),and stand up the tool,test it against something to hold it in place, and let the glue dry. Then if you’re careful,you can turn it and unscrew the screw. Many people use the “super glue” method. Especially on small screws.