Both Game.ini and GameUserSettings.ini are located in,
Filepath may differ a bit.
On dedicated rented servers, the file path can differ quite a bit. Generally, they can both be found at a similar location like this(Nitrado),
Remember to exit your singleplayer game and your server before editing these!
In some cases, singleplayer ini’s can have permission problems, they can be set to read-only, or your game overwrites the ini after you’ve started it. Check file permissions if your changes won’t work.
Similar for rented dedicated servers, where they can offer a GUI they created themself, they can offer you to edit the ini through your browser, or you can have FTP-access to edit the files directly. Check with your provider!