How do i change GameUserSettings.ini in-game?

Is there a way with console commands or blueprint functions?

And is there any way to change the FOV?

Hi!

You can create needed number of Custom Events and run it from console. Or use C++ code create your own console commands.

https://docs.unrealengine.com/latest/INT/Engine/Blueprints/UserGuide/Events/Custom/index.html