First verify the seal you need is 26mm (trust but verify). Also, be certain the one you order is 26mm. I did both correctly and CAS still sent me the wrong size. No problem, they sent me another (the right one) at no cost except for time.
Removing the screw:
Make sure your screwdriver tip is exactly the right size. That may require grinding or buying one. While your doing that pour some acetone on the upright piston head. Let is soak.
With the correct screwdriver, give it a firm but half hearted try. If that doesn't loosen the screw gotta use heat. Coupla methods come to mind (done it before). A soldering gun tip held on the screw head several minutes, try that or... heat gun waved around piston head (say bye-bye old seal).
One of these methods should do it. Afterward clean screw and threaded hole with q-tips/acetone Apply blue loktite to screw threads before installing new seal.