To add or edit a button simply click the gear icon or the '+ Popup' button.
Once clicked you'll be shown this side bar. Within this sidebar you can name your pop-up and style it. On the right part you will see the live preview of your edits. In the preview screen you can click the '+' icon to add elements within the form. You can also click '+ Element' in the upper right corner.
In the overview that pops up you can select the elements you want to use. Now simply fill your pop-up with for example a heading, paragraph and a button. As shown below. That's it!