you can add only three slides in a free theme. if you want more features such as add more slides, different slide animation, manage slider speed, background opacity etc than you have to buy premium version.
In the WordPress dashboard go to the appearance submenu and click on customize.
After clicking on customizing you will in theme customizer.
than click on Slider Section -> Content.
In the content section, there is a number of fields where you can select a page in the dropdown list.
Click on the Publish button.