Here, jim2point0 has posted Cheat Engine tables for Witcher 3 FOV. Confirmed working on Steam or GOG versions.
To use these, you will need to download Cheat Engine. (Be careful on the install process, there is some "packaged software" that you don't want.)
Once Cheat Engine is installed, you will need to run Witcher 3, and minimize it to open Cheat Engine. This is actually the most difficult part of the process, as Witcher tends to crash when returning to it. To circumvent this, set all graphics settings as low as possible, and enter Windowed mode. This is only temporary.
Run cheat engine, and use the processes selection button
Now find and select Witcher 3 from the list of running processes, and open it.
Back in the main screen of Cheat Engine, Using the Open button or Ctrl+O, open the Witcher 3 Cheat Table downloaded earlier. Now confirm 'OK' to the dialogue that pops up.
In the bottom pane, you will notice a new entry has appeared
Check the box in the 'Active' section.
Return to Witcher, and revert your graphical settings. You should now be able to use '+' and '-' in game to change your FOV.