The Slideshow settings can be found in the media section of the project.  This can be found in the top navigation banner in the project.  Once on this page, select the "settings" or "Gallery & Slide show" button in the upper-righthand corner.  Here you will see how the gallery is displayed; either by a grid format or non-grid format which is a carousel.  You will also be able to change the slide show settings of the type of transition, the speed of the transitions, and the music that plays during the show.  You can also set the slide show to play automatically or manually by the play button.

You can also set these default settings in the Setup>Features page.