If you overscrewed it and now the screw just turns then you’ve probably stripped the thread in the “female” part, but first check the screw itself - can you still see the threads? If yes, then you probably stripped the threads in the body of the phone, which is a pain!
However, before you go any further, will one of the other screws from the phone fit in the same hole? Will it tighten? If yes, then you just need a new screw - I suggest taking it to an electronics repair shop, they may have a spare they can sell you. Failing that, you need to find the screw dimensions to buy a new one. I suggest that you message Fairphone support, but whilst waiting for them to reply you can try and measure it yourself. For this you need a micrometer and a thread gauge which you can get from a tools shop. To use them to measure the screw I suggest you look for videos online.
If another screw doesn’t work then you’ve probably stripped the thead. There are two ways to fix this, neither straightforward:
- “Tap” a new thread into the existing hole. This will then require a larger diameter screw, matching the new thread you tapped in
- Insert a helicoil… this may also require that you tap in a new thread as well, maybe not.
Again, these require tools from a tools shop - a tapping set, a helicoil and maybe some other things… again, best to look online for videos.