Step 1 - Use an online calculator to find out what fov you want and the fov values. Search "Arma 3 Fov Calculator"
Step 2 - Go to Documents, Arma 3 - Other Profiles
Step 3 - Find your active profile (the one you use)
Step 4 - Open the vars file and press CTRL F, search for fovTop
Step 5 - Replace fovTop and fovLeft's values with the calculated ones
Step 6 - Save and overwrite the file, and open ArmA 3 with that profile.
Additional - If you want to switch between default and custom fov while playing ArmA, go to esc > settings > display settings (i think that's what its called) > the 2nd tab > aspect ratio > 16:9 / custom
(this is from memory, so apologies for any mistakes)