you can add only three slides in 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 WordPress dashboard go to appearance submenu and click on customize.

After clicking on customize you will in theme customizer. 

than click on Slider Section -> Content.

In the content section, there is the number of fields where you can a select a page in the dropdown list.

Click on the Publish button.