Early PC versions of the game didn't have a FOV slider, so you probably have the correct version 1.0.0. To change the FOV you need to go into the game's CFG file which should be located here C:\Users*(your user)*\Saved Games\Arkane Studios\Prey on your PC. Then you open the "game" cfg file with notepad and look for the row that says cl_hfov = 120.000000. You can just change the number to your liking.As you can see I have it set to 120.

The game should be closed while changing the CFG file.

Hope that helps. :)