If you know any local iphone repair shops bring it to them, they most likely will be able to get it out.
Until then they are some methods to use-
1. Get a very small rubber band ( one that would fit between the screw + the screw driver ) and it might have enough force to turn the screw.
2. Get some super glue ( make sure it works metal to metal ) and apply a VERY small amount, you dont want it to it in the internal components. Let it dry, then turn the screw out.
3. If the screw is already out a little, you could probably use some type of tweezers, or something with grips to get it out. Also I would not try to make it worse then it already is.
4. If it comes to it, you can try and drill it out ( i would recommended trying all the above before that ).
Ifixit has some iphone screw sets to replace the screw.
Next time make sure you have the right sized screwdriver, and apply enough force ( but no too much ) so it doesn't slip and strip the screw.