You should be able to attach a steel pin (such as the shank of a small diameter drill bit) to the head of the screw with a bead of metal epoxy (such as JB-Weld) available from most auto parts stores.
Once set, this should allow you to remove the screw and then replace it.
If you think any of the other screw heads are getting worn or distorted, make sure you replace them too. And always use the correct torx driver (never an allen wrench) - #6 for Nokia cell phones.
Good luck ;)