Open the text file named "common.txt" located in the following folder:

Steam\steamapps\common\Conarium\Conarium\Content\Local

Once inside said .txt file, simply change the number listed under <gfx/fov> to the FOV you'd prefer instead (the default value being 80).

Afterwards, make sure you remember to press SAVE CHANGES in Notepad before (re)starting the game.

Ta-daah - your new FOV will be instantaneously apparent upon loading into the game itself (new and saved alike).

AFAIK, whatever value is set should be persistent (until overwritten by future patch/validating game files/reinstalling/meddling nautical-looking relative).

For the discerning gentleman

If in doubt: try increasing the above listed number from 80 to 100 before testing out your 'new Innsmouth look' in-game.

• If a value of 100 makes your field of view seem 'too inclusive' (i.e. like a fisheye lens, no pun intended), try dialing it down to 90 instead.
  
• If you still find yourself needing more past 100, try raising it further still, to 110 and beyond the mountains of madness