All of the config / .ini files that you should be tinkering is within the User Account folder not the actual Steamapps folder. These config files are usually found in the My Documents, unless you put these folders on other places. Which means, you are already an advanced user that needs no further directions on where to find these things.
For Windows users, the easiest way to do this is by searching for the ‘Xcomgame’ folder. Just search it over through your computer’s Search function. This should be easy on Windows 10 and Windows 8 / Windows 8.1.
By default, the files needed are in these location:
%USERPROFILE%\Documents\My Games\XCOM2\XComGame\Config\
-or-
My Documents->My Games->XCOM2->XComGame->Config
For those playing in Mac machines the default address should be,
~/Library/Application Support/Feral Interactive/XCOM2/XComGame/Config/
For Linux / Steam machines (though this is still wildly variable to what distro you are using)
$XDG_DATA_HOME/feral-interactive/XCOM2/WritableFiles/
-or-
~/.local/share/feral-interactive/XCOM2/VFS/Local/my games/XCOM2/XComGame/Config/
Just copy-paste these default addresses (do not forget to change the user name to yours) into your explorer bar or whatever your OS uses for addresses.