You have some options here:
Option #1: If you use Riot's UI in the Full Api Reference, then there is a dropdown list for every endpoint to select a different region from:

Option #2: If you build the request URL yourself, then you have to use the specific host from the table for regional endpoints. So, for example, if you want to see the featured games list for the brazil region, then you have to get the host 'br1.api.riotgames.com', add the endpoint '/lol/spectator/v4/featured-games' to it and add your apikey as a query paramater. Like that:
https://br1.api.riotgames.com/lol/spectator/v4/featured-games?api_key=<your_apikey>
That option is described here.
Option #3: Use one of the many api wrappers in your favourite programming language. I myself am using this one in Java. With such an api wrapper you can make a request for featured games like that:
ApiConfig config = new ApiConfig().setKey(<your_apikey>);
FeaturedGames games = RiotApi(config).getFeaturedGames(Platorm.BR1);