Switch not working: Audiotechnica ANC700bt headphones

The switch isn't working, its lost on off integrity. Its sliding rather than switch on switch off

I have taken the earpad off but cannot open the casing to see the switch.

Looks like there are 4 small Allan / Hex shaped screws to undo. I tried a 1.5mm allan key but its too big. Not sure what tool to use.

Any guides on how to check and change the switch please