There isn't a button to change from sub to dub. Most shows on Crunchyroll are sub only. There are a few which are both, and in most cases you'll see the dub version listed on the main show page as if it's a different season of the show.
We're now proud to reveal that Crunchyroll will be home to the dubbed anime we've had in the works for years. Five languages just wasn't enough - we're thrilled to be able to share English dubs once again with the Crunchyroll audience.
You have to find the show page, or all episodes screen for the series, and change the season to the one without any parentheses after the title, then start playing the episodes that appear under that. How you change seasons depends on what platform you are using to watch.
The easiest way is to first put the show in your queue, then go to the show page, select the dub, which is kept as a separate "season", then start the first episode of that in the web player for about 30 seconds, after which you go to your queue in the app and it should play starting from there.
Search under the thread “CR's English Dubbed Anime” to find an updated list of all the dubbed shows in Crunchyroll's catalog. According to the thread, there are over 190 different series that feature dubbed audio.
As others have mentioned, most of the shows on Crunchyroll are Japanese audio with English or other language subtitles. There are English dubbed versions for a few titles -- most of those are listed in the CR's English Dubbed Anime thread in the Anime forum.
Crunchyroll doesn't have any dubs for any of the Naruto series (except some of the movies).
Crunchyroll maintains a subsidiary in Japan for licensing purposes and for production. But as a platform, Crunchyroll is not active in Japan.