As I recall, I used this DIY video to replace mine. Maybe you can glean something from it. I remember those screws being tight and troublesome to remove and replace. But the fix is going on a year ago and the horn is still working correctly. Good Luck.
https://www.youtube.com/watch?v=2WqhZaaXL2I&list=PLrnXp5l-XGyWq8qBzyCgBGPg3PCT0cTJo