That doesn't look like a screw; that looks like the head of a pop rivet. If you want to remove it, you'll have to drill it out with a bit that is just slightly larger than that hole. Pop rivets come in several sizes, but the drill bit for that one is probably 1/8". Carefully drill until the outer ring breaks loose and then you can push the rest of the rivet into the cavity in the frame. When you've done this for all the pop rivets, the screen should come free.

If you want to remount it -- and you haven't damaged the holes -- you can get a pop rivet tool and fix it in place again. If you have messed up the holes and made them even slightly larger, pop rivets won't work and you'll need to screw it in place.

" If you have messed up the holes and made them even slightly larger, pop rivets won't work and you'll need to screw it in place. "

You may be able to get a larger pop rivet or use a washer if you have boogered the hole but it will look worse than just a small simple rivet for sure.

Could possibly be Chicago screws.. If rivets and have to be drilled out, you can use Chicago bolts to replace the rivets.

https://www.mutualscrew.com/department/sex-bolts-screws-18712.cfm

Thank you so much! You're right, they're pop rivets.

Argh what a pain....whoever installed this fly-screen clearly didn't want it to be removed!

Thanks again, Nat

Nat- Maybe it was installed that way as a security measure....