It is a dumb idea to try and fix it yourself. You may: 1. Damage the gun. 2. Make the screw even harder to extract by messing with it. If you have to ask how to remove the screw, you do not have the requisite skills to remove it without damage. And, after removing it, you will still have to contact Ruger to get another screw and they will frequently ask you to send the gun in for repair so that they can be assured that it is going to be repaired correctly.
Ruger has historically, one of the best service departments. They fix what they make. If they damage the barrel, they will replace it.
I they have fixed guns for me that I bought second hand (had no obligation to me), and "fixed" guns that I have lost parts from...without charge.
That is why it is a dumb idea to attempt to fix the broken screw yourself.